Shapeshifter Baggage

""The description for the hammerfist's power says that he focuses hundreds of pounds of biomass into his arms to turn them into wrecking balls. Hundreds of pounds of mass from where? From 'fuck you, I'm gonna smash a tank into pancakes with my bare hands now', that's where!""

- Something Awful poster McSpanky on Prototype

Whenever shapeshifters are depicted with the context of magic they have the ability to assume any form regardless of the mass difference between themselves and the new form. Usually no attempt is made to explain how the shapeshifter disposes of excess mass or acquires needed mass.

But if given a scientific basis, this apparent violation of one of physics' most basic principles is sometimes given a rudimentary form of explanation.

If the issue is brought up at all, it's only to avert the trope by pointedly illustrating that its shifters cannot change mass and/or size. Or they may Hand Wave the issue with some quasi-scientific Applied Phlebotinum, e.g., 'Pym Particles' in the Marvel Universe. It's also possible that the character draws mass from an extra-dimensional source, or sends mass there when they get small (if any explanation is given to magical shapeshifters, it's usually this).

This can also apply to characters with a Healing Factor, as many of them appear to be able to regenerate biomass from thin air. Tropes that avert using Shapeshifter Baggage are Pulling Themselves Together and Appendage Assimilation. Compare Elemental Baggage and see Required Secondary Powers.

Sometimes explained by increasing or decreasing the density of mass, making the shifter smaller or larger by changing the spacing of the particles/darkmatter/etc. that makes up their body. Sometimes it is done by consuming mass like food/drink/flesh/etc. then expelling it later to change size, shape, or mass.

Anime and Manga

 * Averted in Devilman. When
 * S-Cry-ed is a good example of a justification of this trope, although transformation is only seen in one character and in a subset of the typical creation power. Whenever somebody creates something, the mass they require for it is taken from surrounding objects or the environment itself. When the aforementioned character creates his robot arm he not only melts down half of his body, but a large part of the landscape and, the first time he gets it, a bird flying overhead. Yikes.
 * Taken up a notch with an Alter User who created a Humongous Mecha. His first use of it, in response to a bunch of villagers backing him to the edge of a cliff, was to vaporize them as material.
 * In Kenichi Sonoda's Cannon God Exaxxion, the Robot Girl sidekick can shapeshift into a hoverbike. She stores the excess mass needed to change in her breasts.
 * Averted in the Fullmetal Alchemist manga. Envy, despite its lithe and feminine physique, is very heavy, and as a result, makes very deep footprints and can break a steel fence after a short fall. This is noticed by the main characters and clues them in on the size of Envy's true form...
 * Subverted in the lesser known manga Momoiro 1/10 -- The titular girl is 50 foot tall and weighs several tons. When she's shrunken to the size of a normal human, she still weighs several tons and is denser than lead. Trying to walk across the top floor of her school causes the floor to collapse, running across a parking lot causes a trail of destroyed concrete, etc etc.
 * The first chapter of Mai Chans Daily Life shows that Mai's Healing Factor is fueled by organic matter. In this case, a butchered chicken stuffed into her abdominal cavity.
 * The Pokémon Ditto can change into 'mons of any size, but the anime once subverted it with a Ditto that couldn't change size.
 * After the Magic Pants, this is the second most common source of Fridge Logic in Ranma 1/2 whenever Jusenkyo transformations are concerned: Mousse, Shampoo, and Ryouga all change into smaller animals with no explanation as to where their mass goes, or where it comes from when they change back. Genma and Pantyhose Taro change into larger animals (in Taro's case, a chimera that is several stories tall.) On a lesser scale, Ranma, Herb, and Rouge also lose or gain mass drastically, even if they remain roughly human-shaped (in Rouge's case) or merely change sex (for Ranma and Herb.) Might be justified in that Jusenkyo curses its victims with magical transformations.
 * Saikano uses this, but doesn't attempt to handwave it or anything of the sort.
 * Boomers in the Bubblegum Crisis series present this problem. How can a man-sized android shaped like an ordinary salaryman sprout enough mass to turn into a Humongous Mecha with flailing tentacles?
 * At least in one case it was because it was an absorbing type Boomer that used nanotechnology to fuse machinery, and eventually even scenery to its mass.
 * The Invaders in Shin Getter Robo Armageddon. For example, in one episode, the main characters respond to a distress signal from the ruins of New York City. When they get to the origin of the signal, they find two puppies and a dead person at a radio station, but the puppies afterwards turn out to be a pair of building-sized Invaders from a larger group that set a trap.
 * The Saiyan Oozaru form from Dragonball is never fully explained; how does one quickly transform from a human-sized alien to a full-blown giant monkey? Of course, not much else makes "scientific" sense in that series.
 * However, Oolong the pig's physical strength and body mass remain constant regardless of his current shape. Simply becoming a motorbike, for instance, doesn't guarantee he's strong enough for Bulma to ride.
 * Happens often in Naruto, as it seems chakra has No Conservation of Energy. One of the biggest offenders is when 's true form is revealed when he spits out a baseball-sized piece of something through his throat... which grows into a monster several times the size of his body.
 * The second biggest offender is Kimimaro, who can grow bones from his body and use them as weapons, apparently just by controlling the hormones of his body. Putting aside that he apparently needs to be able to survive removing his own bones without bleeding to death this would also require his body to have an infinite amount of calcium to make the bones out of.
 * The manga Kimimaro seems to have an accelerated healing factor which attempts to explain this, but really just invokes the "Biomass from thin air" part of this trope.
 * In the manga Gantz, the final boss of the Osaka mission "Nurarihyon" could change shape and size from a little old man to a monster the size of a high rise building, and is capable of splitting to multiple variable life forms of himself and demonstrates different powers with every different forms he assumes. He could survive being squashed to a puddle of blood and reform to another different gigantic monster instantly.

Comic Books
"Yeagar: Does anyone know why our fifty-foot henchman was just whisked into the sky by a thirty mile-an-hour breeze?
 * The Hulk.
 * The Ang Lee film made some attempt to justify it, basing Bruce Banner's initial transformation on real animals that appear to "Hulk Out" (i.e. get bigger) when they are scared or angry. Ok, cool. Then it turns out that the Hulk grows even bigger when he gets even angrier...
 * The Other Wiki's article on the Hulk states that gamma rays, being the most powerful form of energy known, are so strong they can actually transform energy into physical mass, possibly explaining the increase in size. Now, as for where the mass goes when he transforms back, that's anyone's guess.
 * In the Marvel Universe, a common explanation for size increase/decreases/etc is that characters' added/excess mass is "shunted off into a pocket dimension."
 * Similarly, the censors are mollified by having uniforms made of "unstable molecules" for most shapeshifters, either heroic or villainous. The example springing most easily to mind is Rahne "Wolfsbane" Sinclair, of the original New Mutants class, whose skintight school uniform shifts into a collar when she turns into her wolfoid and wolf-forms. A collar that is so small that it doesn't even distort the fur around her neck, but is so comfortable that she has never, ever mentioned its existence.
 * Rahne was frequently 'sartorially challenged' in the early comics she appeared in. Yet more than once changed from plain clothes to super uniform between panels.
 * Averted in the novelization of X2: X-Men United, where Mystique reflects that she's good enough that she makes it looks easy -- even though it's not. If she wants to grow in size significantly, she has to gain mass; if she wants to shrink significantly, she has to lose mass; and always she has to keep in mind the position of her organs.
 * Based on the comics (unless this changed recently); unlike other shapeshifters, Mystique can't change mass and has to keep a generally human form. It's even suggested that she can remain in her Raven Darkholme form indefinitely because there's no change in mass, while trying to keep the same mass in a larger form is a strain that keeps her from using such forms for long. Given her age and experience, however, that sort of 'strain' is a relative term. In her solo series of comics, she demonstrated an incredible creativity with the use of her powers despite the limits she had compared to most other shapeshifters. Among the feats she performs is flattening out to avoid taking damage by an explosion, assuming a monstrous form by sprouting a second face and pair of arms in order to quadruple wield (though she specifically mentions that doing so was very dangerous to her health, brain, and a massive strain to her powers), faking a glass vial being broken in her hand, faking the effects of a lethal virus, smuggling items in a "flesh pocket" in her stomach, and pointing out that since she mimics all her clothing too, she's technically naked all the time.
 * Slightly (and disgustingly) avoided by Big Bertha of the Great Lakes... um, at the moment, Initiative. It's not explained where the extra mass comes from when going from supermodel Ashley Crawford into I-stop-traffic-the-hard-way Big Bertha. But after the heroics are done, she does have to expel the extra mass... by vomiting it back out. She doesn't like it any more than you do.
 * The alien Skrulls are explicitly described as being unable to alter their mass, and therefore having size restrictions on what they can imitate. Some writers forget this, as when a Skrull character became a flea to hide on someone's person (or in their very first appearance in Fantastic Four, when a Skrull poses as Sue Storm and mimics her ability to turn invisible by shrinking), or gained tremendous weight to crush someone.
 * This is "explained" by the existence of mutant Skrulls with the powers to do things like their human counterparts.
 * Averted by The DCU's Martian Manhunter. In an issue of his solo series, he reveals that he can borrow mass from the Earth.
 * He also dodges it in that, since he can make any part of himself intangible and invisible, he can hide any excess mass he's not using.
 * Micro-Might from PS 238 averts this trope. When using her powers, she gets smaller, but not lighter; she gets denser instead, apparently using the Square-Cube Law (although in a comic book physics way) to get stronger and tougher.
 * Mass Master of Power Pack also had this property, and may have been the inspiration for Micro-Might.
 * Subverted in Nodwick #4. Nodwick drinks a magic potion that makes him grow to about fifty feet tall, but is then blown away by a breeze.

Artax: I might have an answer for you...

Artax: According to the fine print for "Plan #1", the growth potion increases volume, but not mass."


 * In Ultimate Iron Man, Tony's Healing Factor lets him regrow his legs, but he has to become a Big Eater to get the necessary materials.
 * Spider Man's Doctor Connors regrows his arm whenever he becomes the Lizard. When this transformation happens gradually it can be Handwaved, growing it back was, after all, the point of his research. But then there's Komodo, whose improved version of the Connors' formula lets her shift between forms at will. In human form, she hasn't got any legs, and it becomes a bit harder to rationalize someone losing and growing back their legs as a result of falling asleep on a plane for a few minutes.
 * Atom Smasher of the Justice Society of America received his powers from his radiation-affected grandfather, but he uses a surprising mechanism for growth. His bones and muscles literally break and regrow as they stretch, pointing to some sort of accelerated biological growth process.
 * Warlock seems to have no limit on how large he can transform (and change his mass to fit the size) instantly, but for some reason he doesn't really transform into anything smaller than he originally was.
 * Transformers is explained in greater depth down the page in Animation, but the comics have the same issues with Megatron transforming into a gun and Soundwave and Blaster as cassette players, all about big enough to fit in their hands in robot mode. Possibly the most egregious offender of all is Broadside, a triple-changer with alt-modes as both an aircraft and an aircraft carrier.

Fan Fic
"You turned into a cat! A SMALL cat! You violated Conservation of Energy! That's not just an arbitrary rule; it's implied by the form of the quantum Hamiltonian! Rejecting it destroys unitarity and then you get FTL signaling!"
 * Lampshaded in Harry Potter and The Methods of Rationality: as part of McGonagall's Magic Test, she demonstrates her Animagus abilities. Harry immediately flips the fuck out at this blatant violation of the laws of physics.

Film

 * Averted in Terminator 2. The evil terminator, the T-1000, can shapeshift, but it is explicitly stated early on that he cannot turn into anything with mechanically complex parts nor can he gain or lose mass. Pedants have pointed out that the human body is very complex, but shots that show the T-1000's interior reveal nothing but solid silver-colored metal. In essence, if it can mimic behavior or the exterior shape of something, it will do that as opposed to trying to copy it exactly. The T-1000 could create the outer shape of a gun but couldn't create an actual, working gun. Much like a ball of clay; the outside matches, but the inside isn't the same as what it's mimicking. Also, it is obvious in the movie that the T-1000 actually moves by morphing, not via anything resembling human structure (at one point, he’s held with his face to the wall, and he simply morphs his back into his front; at another, the T-800 puts a fist through his head, which promptly becomes a hand holding said fist  while a 'new' head sprouts from its shoulder). Further still, at one point in the novelization, the T-1000 takes the shape of a fat policeman, it is explicitly said that he “didn’t like the shape”, because its larger volume forced him to assume a less dense configuration (presumably, he morphed “bubbles” inside himself); it explains why he repeatedly returned to the “thin” policeman shape: it was just right.
 * John Carpenter's The Thing is mostly fairly good with this, with The Thing usually seeming to obey conservation of mass. However, at the end the is able to turn into a quite large monster. In fairness, he seems to have attacked another man first, and it's possible he consumed additional material since the transformation happened offscreen.
 * Species starts out fairly good with this, with Sil having to eat huge amounts of food to maintain her bamboo-like growth rate. Then the realism level crashes and burns when she goes into her cocoon and comes out a mature woman who has to weigh about twice as much as the little girl that went in. There's an attempt to justify it by explaining her food consumption as "storing up calories for some big event" except if that was true she should have been visibly bloated from holding her own weight in food in her stomach, and she wasn't. The whole cocoon process was probably mostly just made up so Sil could go straight from asexual child to mature sexpot without a logical but family-unfriendly intermediate stage. They repeat this with, whose rapid growth rate is fueled by gobbling up sewer rats, but who then becomes visibly larger without any proportionate mass intake when he assumes his One-Winged Angel form.
 * This last is somewhat averted in that the transformation of  wasn't exactly neat, tidy or complete. One limb is far shorter than the others. The muscles are stretched very thin. Bones are exposed. There isn't enough skin to keep everything covered. In fact, the internal organs hang from a gaping hole in the abdomen. It's what you'd expect to see if a small amount of mass suddenly tried to expand into a larger shape.
 * A more egregious example of the filmmakers' disregard for the laws of physics and basic biology is the  the protagonists create in an attempt to better understand Sil. From a single cell, it erupts like a fast-growing fungus to fill almost a whole room, this without any apparent source of food or even water.
 * And let's not forget that ... thing ... that infected astronaut turned into at the end of Species II. The transformation is off screen so I suppose it's technically possible that he paused in his make-out with to gobble down a few hundred pounds of burgers, but seriously, come on.
 * There's a good reason why, in the comics and cartoons, Spider Man used web-shooter devices. The movies don't hold to this tradition, however, and no explanation is given as to how his body can produce and secrete so much organic matter without turning him into an extreme Big Eater.
 * The X-Men series of films:
 * Wolverine's fast healing must take a hell of a lot of stamina/metabolism.
 * Moreso with the guy from Last Stand who could shift body shape at will and re-grow arms (but not all his anatomy) within a fraction of a second.
 * Mystique can apparently take on appearances with variable mass, as seen with her shapeshifting into a little girl on that prison truck.
 * The guy who could grow bone-horns and throw them
 * Multiple Man.
 * Where did Beast's fur go when he walked near Jimmy/Leech?
 * The Incredible Hulk: Where does all that sudden biomass come from? And where does it go?
 * In the first Hulk movie (Not the one with Norton), Every time Banner is seen returning to human form, we see steam emanating from him and water dripping from every pore of his body, slicking his hair to his head and soaking the Magic Pants to him. Presumably the excess mass is created by rapid cell division (That's where it comes from for normal humans, after all) and when he calms down, the cells are turned into steam/water and are shed thusly. Wouldn't be the strangest thing to happen in a Marvel movie.
 * The Hulk's greater size, when compared to Bruce Banner, might be explained by him simply inflating himself like a pufferfish -- i.e. he gets bigger, not heavier -- except that when he's shown strapped to a table and deliberately induced to transform, the support strut for the table buckles under his extra weight.
 * The Star Wars Extended Universe features the Shi'ido, a species of shapeshifters who are nosy about the affairs of others, but keep to themselves when it's time to be sociable. Evidently, the problem of changing mass (one "encyclopedia" style book includes an excerpt from the logs of Senior Anthropologist Hoole, who describes how, studying a world of creatures about a meter tall, he saved a pair of them from a storm by transforming into a Wookie and using his greater mass in this form to hold them down) is one that they learn to overcome around age 150, but how they overcome it doesn't come up, apparently.
 * Honey I Shrunk the Kids attempts to answer this question, but in fact just raises further questions with its explanation. Reducing empty space does not reduce mass, after all. One shudders to think what it must be like to weigh 120 pounds at a quarter of an inch tall.

Literature

 * Originally in Animorphs, the size/mass changing was essentially Handwaved by explaining that a character's DNA is "rebooted" every time they transform. But in a later book, Ax explains that morpher's extra mass is stored in a pocket universe when s/he is in a smaller form, or taken from there when in a larger form... which also happens to be the hyperspace bypass that starships use. He even points out that the extra mass has to go somewhere. There's a Million-to-One Chance of having one's stored extra body mass run over by a random spaceship. Needless to say, the Animorphs are less than thrilled with this info. This actually happens in one book, whereupon the main characters (who were mosquitoes at the time) connected to that mass are "slingshotted" onto the ship... and take several minutes to put themselves back into one piece. With disgusting results for the onlookers. (The Animorphs aren't too interested in hearing about how they rewrote the Andalite science textbooks.)
 * Played straight, however, with the Helmacron "shrink ray". Animorphs and Yeerks are shrunken and unshrunken with impunity, without even a Hand Wave as explanation.
 * The Shrink ray technology came from manipulating the morphing cube that gave the kids their powers. So presumably the Helmacrons used the same pocket dimension mass swapping technology just for a permanent shrinking effect instead of a temporary morphing one.
 * There is still the question though of where the extra mass comes from when someone morphs into a bigger form.
 * Is there really?
 * Applegate explained once that it came from "Black holes, and pygmy hippos."
 * Averted in Poul Anderson's Operation Chaos where all animal based lycanthropes must observe Conservation of Mass. The 180 pound hero transforms into an 180 pound wolf, while a Giant Mook who turns into a tiger is "seven feet tall and monstrously fat."
 * Harry Turtledove not only keeps to the principle (illustrated distinctly by a couple werehawks too heavy to fly) in Werenight, he gives this particular Giant Mook a semi-affectionate nod... and upgrade. An "immensely tall, immensely fat" barbarian chief turns into a sabretooth. A big sabretooth.
 * This is averted in some Discworld books. In one book, a character who has been transformed into a toad wonders, "what happened to the rest of me?" In the very next book, we learn exactly what happens and it ain't pretty. In other books this doesn't happen, though. Essentially the Shapeshifter Baggage question raises a conflict between two fundemental Discworld laws: the Law of Narrative Causality (which says it should work like it does in stories) and the Law of Conservation of Reality (which ssys it can't be that simple). It's also possible that it depends on how the shapeshifting occurs; "natural" shapeshifting and Baleful Polymorph via mental tricks and morphic fields have "neater" results that using magic to directly warp someone's shape.
 * Vampires don't have to follow mass conservation rules, but they find it easier to turn into many bats rather than a single bat, especially if they've been off human blood for a while. And because the Discworld's Genre Savvy universe understands what Fan Service means, male vampires can shapeshift with their clothing, while female vampires can't.
 * In the Star Wars Expanded Universe (in particular, the "Galaxy of Fear" series), the race known as the Shi'ido can shape-shift. It's explained that due to their extremely long lifespan, in which they can live for 500 years and only people older than 61 are considered adults (makes you wonder what their drinking age is), their shapeshifting ability improves with age. Young Shi'ido can only change skin color, but older ones could change into any humanoid species they wanted. However, if they tried to change into aliens beyond their natural boundaries, they'd be stuck in that form for weeks or months. How exactly they could change isn't well explained, but The Essential Guide to Alien Species said something to the effect (if this editor remembers) that they have folds of extra skin under their skin that they can use if they need to to change into larger or smaller species (or rocks and trees, apparently). Also, since some species identify others with smell as well as sight, and the Shi'ido aren't perfect at what they do, it's explained that they use telepathy to get around (what could be described as) Latex Perfection.
 * Handwaved in The Elvenbane, wherein the dragons explain that when they change from huge firebreathing beasties into humans or elves, the extra mass goes into an extraplanar space they call the Out. (A similar conceit is implied in the D&D universe, even though the transformations are magical.) A dragon in human form, when viewed in the magical spectrum, can be seen as a human surrounded by a dragony "shadow".
 * Averted in Dean Koontz's novel Phantoms. The Big Bad monster can change its shape and detach pieces of itself, but it must obey conservation of mass. Its creations often appear outsized when it tries to imitate something too small or too be big, and it's only able to imitate a much smaller creature by splitting itself apart.
 * In the Fafhrd and Gray Mouser novel The Swords of Lankhmar by Fritz Leiber, a shrinking potion does, in fact, displace mass, as the now rat-sized Mouser has to swim his way out of a good-sized puddle of meat, cloth fibers, and metal fragments (flesh, clothes, armor, and weapons). Later, he grows back to his full size away from that puddle, and the mass is taken from nearby objects (and people!), notably a very fat girl who finds herself suddenly slim. Great news for her, Squick for Mouser?
 * Averted in the Wild Cards series, where Kid Dinosaur can change shape into any kind of dinosaur, but explicitly does not change mass. This results in such things as a 3 foot tall T-Rex.
 * Played straight with another character who's body stored everything he ate (he never had to go to the bathroom) and kept absorbing until he had enough mass and excess food to go into a prolonged hibernation, during which his body would radically change (as would his powers).
 * Skewed with Rahda "Elephant Girl" O'Reilly, who is a Irish-Hindu were-elephant. Her excuse is that she absorbs energy from the environment and converts it into mass; this can black out a city if used in the right location. Likewise, when she changes back the excess mass converts into a flash of light. Of course, the amount of energy needed to convert into a couple of tons of elephant flesh is incredibly titanic; and the energy release from changing back should wipe out a continent. So it's neither averted nor played straight.
 * The Timeweb trilogy by Brian Herbert takes the more obvious approach: shapeshifters grow larger by absorbing rocks and dirt into their own mass. Growing smaller is somewhat like shedding snakeskin, and can be a bit disgusting if a massive change is needed.
 * Bit of a family tradition, this. In Frank Herbert's novel, "Man of Two Worlds", a shapeshifting alien is captured by humans and is confined to a cell with only a small drain being the way out. He laments the fact that he can't simply destroy his own mass so that he can become small enough to fit through the drain. Too late does he realize that he could have just turned into a snake and slithered down the drain, without having to bypass the law of conservation of mass/energy.
 * Used and Lampshaded in The Shapeshifter book series, Dax Jones can turn into a fox, but has no idea where his clothes and whatever he is carrying disappear to when he does.
 * Averted in Sector General, where Dr. Danalta does not, in fact, lose or gain any mass when it changes shapes...so when it turns into something that looks like a teddy bear, it is still a very heavy teddy bear.
 * Worried about and obsessed over in the Whateley Universe: ordinary Shifters can't violate Conservation of Mass, but the highest-level Shifters (who may not be using the same underlying principles) can. The researchers are still trying to figure out where the extra mass goes or comes from, but it seems to be inter-dimensional. As for clothes, the best Shifters shift their own, and have to learn how to do it right so people aren't pointing out that their 'dress' has pores and hairs showing.
 * Featured regularly in Jim Butcher's The Dresden Files. Conservation of mass isn't a problem at all for magical shapeshifting thanks to ectoplasm. When a creature shapeshifts, or when something from the Nevernever (faerieland) comes to the real world, the matter (in the case of a shapeshifter, the extra mass layered over their real body; in case of a faerie, their whole physical body in the real world) is formed out of ectoplasm from another dimension, animated and given substance by magic. When that magic is withdrawn, the ectoplasm turns into an equal mass of a inert, clear, viscous goop which is an inconvenient mess but quickly evaporates. Arguably justified, in that there's an explanation of and consistent rules for where extra mass comes from that make as much sense as anything else in the series: leftover ectoplasm has been used to identify a crime scene as magical in nature, and I'm sure that characters have slipped and fallen on the stuff at some point or other.
 * This doesn't seem to explain how shapeshifting into something smaller works, though. Where does the extra mass go? The Nevernever? If so, how is it protected from some nasty spider-goblin thing that probably wants to eat it?
 * According to the official RPG, that's exactly what happens. The margin notes from Harry and friends specifically mention this as a good adventure hook.
 * Explicitly averted in the Kitty Norville novels. The easiest way to distinguish a werewolf in lupine form from its mundane counterpart is the fact that they are normally at least half again as large as the 36 kg (80 lbs) norm.
 * Steven Erikson's Malazan Book of the Fallen has this feature for both Soletaken and D'ivers (single- and multiform shapeshifters, respectively). Depending on which one of these beings you encounter, you might be up against a grown man who can become a hawk and fly away... Or something that can become one or more dragons. At least the undead shapeshifter can't become living...
 * Katherine Kerr's Deverry series, despite being magic based, required Dweomer workers who change shape to retain their mass. Making them quite large birds.
 * Kelley Armstrong's werewolves in her Women of the Otherworld series retain the same mass in either form, and have to eat lots to account for their higher metabolism.
 * In Sheri S Tepper's "Mavin Manyshaped" trilogy shapeshifters can increase their mass by incorporating additional organic material (Mavin uses a sack of grain at one point) but decreasing their mass (beyond discarding the additional material) is never addressed.
 * It is mentioned in one of the other True Game world's books, though; Peter mentions that when he decreases his mass the excess is expelled, resulting in what looks very like a pile of minced beef. The significant Squick factor tends to stop it being made into burgers, though.
 * In Mercedes Lackey's and Andre Norton's Elvenblood series, where shapeshifting dragons shift excess mass into the "Out", which is implied to be Another Dimension. Not all dragons are equally skilled, so only the best can assume radically smaller forms such as humans and elves.
 * In the Jane Yellowrock series, by Faith Hunter, Jane is a Skinwalker, capable of copying the genetic code of animals and possibly people to assume a new form. This native american magic allows her to sloth off mass and store it 'else-where' (mainly stones and sand) and to gain mass to grow in size. Interestingly enough Jane likes to only absorb or deposits mass into stone because to her it doesn't have any individual traits aside from being empty matter.
 * In Liar by Justine Larbalestier this is explicitly averted; werewolves are exactly the same mass in both forms.

Live-Action TV

 * Odo from Star Trek Deep Space Nine is described as "heavier than he looks" (while in human form), which would make sense if he spent most of his time getting bigger instead of smaller. Apparently he can be heavier than an average human and still manage to turn into a bird and fly around (remember, "heavier than he looks"), or be carried around by Rom easily while in the shape of a glass.
 * Apparently, he can also change his molecular and atomic structure. When he shapeshifts into a wall, even sensors can't tell him apart from the real thing.
 * Peter David's "The Siege", the first Deep Space Nine book if we don't count episode novelizations, had one of the characters literally taken off their feet by having their pantleg grabbed in the teeth of a 180-some-odd-lb mouse as they're (trying) to run by.
 * Odo CAN change mass, as stated in "Broken Link". In Season 7, we also learn why: Founders can access the subspace for FTL. Most likely this access gives them mass-lightening abilities.
 * Star Trek has a long proud history of shapeshifters who seem to tell the First Law of Thermodynamics to take a flying leap. The shapeshifter in Star Trek VI had no trouble changing from a small girl to a large furry creature to a duplicate of Kirk, and the Allasomorphs in TNG's "The Dauphin" were able to double their size and reverse the process with no apparent change of mass.
 * To be fair, there was no indication that the shifter in Star Trek VI changed anything but shape, as all they did while large was look intimidating, and while small simply slipped out of chains. Their mass could have remained the same the whole time.
 * In the original series episode "By Any Other Name", a Vulcan Mind Meld revealed a human looking race to actually be "Immense beings... 100 limbs which resemble tentacles, minds of such control and capacity that each limb is capable of performing a different function."
 * The Doctor from Doctor Who changes size between regenerations, and it's never even brought up where the extra mass goes when changing his form.
 * In "The Daemons" the alien Azal creates vast amounts of heat whenever it changes size.
 * Or cold. Presumably, heat is created when it loses mass by shrinking, and is absorbed when it gains mass by expanding. Though, the amount of heat generated by getting rid of enough mass for a 20 ft monster to turn microscopic would make quite a mess of your planet.
 * In "The Lazarus Experiment", Professor Richard Lazarus exposes himself to a de-aging device which mutates him into a giant, life-force devouring monster. However, despite the radical change in size, it's stated that he hasn't actually gained any mass and thus his body is highly unstable.
 * At first Heroes kept it fairly reasonable. The series first "shapeshifter" was really a Master of Illusion. In the fourth volume they introduce a genuine shapeshifter, whom power thief Sylar promptly snacks on. At first, Sylar only used his shapeshifting power to shapeshift into people of roughly the same size and weight as him (and also still wore the same clothes before and after shifting). However, after a few episodes, Sylar is able to shapeshift his clothes, and also turns into a kid.
 * In H₂O Just Add Water, the girls apparently turn into mermaids and back by storing their other body in some kind of Shapeshifter Baggage. In one episode, Cleo is wearing thick clothing when she is knocked into a pool, then she turns into a mermaid wearing a single bra, and when she turns back into human she once again has the thick clothes on. It also works the other way: Emma wants to dye her hair red, and as this requires water, she does it in mermaid form. Then she becomes human... with blonde hair. Later she goes for a swim and once she's a mermaid she has red hair again.
 * In Supernatural, shapeshifters are shown to shed their old skins whenever they take on a new form. This only covers half the issue, though.

Tabletop Games

 * GURPS gives a nod to this. Shapeshifting normally lets you gain or lose mass however you wish but the Mass Conservation limitation stops this and goes a bit further noting that 150lb mice and elephants shouldn't be allowed.
 * White Wolf's Werewolf: The Forsaken takes time out in a supplement to explain the deep mechanics of Uratha (werewolf) biology. Among the various points is the fact that Werewolves don't actually shift shapes... they swap shapes. Uratha meta-biology has all five of a Werewolf's forms existent at all times; one in the physical world, and four stored as spiritual energy templates in the Werewolf's aura. As a result, beings that can see Auras perceive Werewolf auras as being intensely bright and dense, with the light brightening or dimming based on the size of the physical form. That means the aura is brightest in Urhan (normal wolf) form, and dimmest in Gauru (Man Wolf Death machine).
 * Although, if you're paying attention to the aura while in the presence of an eight foot tall enraged evolutionary monstrosity honed for violence... one might question your priorities.
 * Several dragons can take an alternative form of a medium size animal or humanoid. The smallest shape-shifting dragon can weigh 1/8 lb or less, the biggest can weigh 125 Tons or more.

Toys

 * The Makuta from Bionicle are actually Energy Beings wearing shapeshifting armour, which has a mass limit. However, they can absorb other living creatures in order to make the suit of armour bigger.
 * When not in use, they can store excess mass in a Pocket Dimension.

Video Games

 * Suika Ibuki from the Touhou games has the power to control density of things. Among other things, she can use it to grow to an enormous size or turn into mist (even into a cloud of mist that envolved the whole region).
 * What's especially weird is that she can also split her mass into ... small, complete versions of herself. At this point it's easier to remember that ZUN is frequently drunk, and that putting more thought than he does into the subject is an exercise in futility/hilarity.
 * Well, clearly, she's increasing the population density of the surrounding area, by making more people to inhabit the same area.
 * The spirit channelers of the Ace Attorney series change their body shape and size to what the channeled spirit looked like when they were alive. Many mechanics of channeling, including the question where the extra mass comes from (or goes to) go unexplained in the series.
 * Channeling is tantamount to magic, mind you.
 * A pretty egregious offender is the Resident Evil series, where injecting oneself with any of the mutating syrums tends to cause the subject to double in size and sprout all manner of tentacles and spikes. In Resident Evil 5,  goes from being about 5ft tall to twice the size of a blue whale in about 10 seconds.
 * When fighting Gandrayda in Metroid Prime 3: Corruption, she transforms into multiple characters you've already fought over the course of the game. Most of what she transforms into is sensibly around her own size, with the notable exceptions of Ghor and his ginormous powered-armor, and the massive Berserker Lord. Her scan page states that how she can change size so drastically is unknown.
 * X-Parasites also tend to do this while mimicking creatures. Most creatures of roughly human to twice human size will require just one X, despite the X only being about the size of a basketball. Some other creatures, like certain Space Pirates, will require two or more (Golden Pirates have five for some reason) and bosses will generally have a larger Core-X surrounded by smaller X, showing at least a semblance of realism.
 * On the subject of Metroid, where exactly does Samus stuff her body when she goes Morph Ball?
 * This is at least lampshaded in Metroid Prime, where one of the scans in the Phazon Mines reveals that the pirates attempted to copy morph ball technology, but... well, the results weren't pretty.
 * Prototype averts this rather well. Size-wise all the people Mercer absorbs are about the same size as him. It's also strongly implied (though never stated) that Mercer weighs significantly more than a normal person does, which gives him plenty of mass for his larger powers (such as his whip arm or shield form) or denser powers (hammer fists, as in the page quote, and armor).
 * Also, whenever Alex uses his non-shapeshifting powers (like super-strength or clinging to walls), there's an effect that seems to imply mass is being expended. We see a similar effect when he's gliding. This seems to imply that he's "burning" the extra mass, keeping himself from getting too heavy.
 * Try standing on a vehicle - most cars and other vehicles will be unable to move, and even tanks are slowed down by his weight. Using thermal vision, Alex is also far, far warmer than other humanoid entities (such as civilians, soldiers, etc.). Whether this is to make him easier to see and control for the player or yet more suggestion that he has much more mass is unknown.
 * In thermal vision, Infected also appear brighter than normal civilians and even soldiers, which could imply that, like Alex, they are far denser than ordinary people.
 * One has to wonder that if Alex has all that mass to create craters from very high falls and elbow drop with a heavy hammerfist to shatter armor among other things, how is it that he can flip(by tapping the jump button) onto a normal car(from the same level) and walk onto its roof and not have it crushed under what is supposed to be exceedingly heavy weight over a small area(a pair of feet)? After all, to throw an army truck without shoving himself in the opposite direction, he needs to possess a comparable amount of inertia himself, hence a cab-crushing amount of mass.
 * Unless of course he uses biomass to "adhesively" secure himself to the ground before throwing stuff that should still weigh a lot more than him. Doesn't really explain air-throws though.
 * As long as he takes off his shield and armor, he can sprint as fast as a car and still trivially knock people down non-lethally... anyone?
 * Even better: Ever tried shoving someone right next to a deformable object/terrain? What should been a shove that pushes someone moderately, if you miss, can dent cargo containers, vehicles, even a military base exterior fortification wall!
 * Project Eden has rats that transform into monsters over twenty times their original size, then then turn into a small splash of green goo when they die.
 * In one, maybe not as egregious version as some of the above, we have Left 4 Deads tank. According to the game, it takes place just a few weeks after the first infection. Yet, you regularly meet up with the thing pictured above.
 * In Halo, the Flood has a pure form (which aren't based on an individual infected being) which can change its shape for various functions. Interestingly, the mobile form is quite small and light, but it can change into a large "tank" form in a few seconds which is significantly more massive, and its shooter form should be losing mass as it shoots, but it has no effect on its ability to change again.
 * Averted in Mega Man X with Axl, he has a copy chip that allows him to transform into any Reploid he has a genetic sample from, as long as said reploid is aproximately the same size as him, however it's played straigth with all the other new generatioin reploids (from wich Axl is a prototype),some of which can, all of whom are of different sizes.
 * Anybody injected with the TITAN drug in Batman Arkham Asylum and Batman Arkham City grows to a huge size, The Hulk-style. Where the extra mass comes from is never explained, nor is.
 * Clayface in Batman Arkham City is a gargantuan monstrosity, towering well above anybody in the game including TITAN henchmen and Bane. And yet . He could theoretically be completely hollow inside, but then his hammer-hands attacks would probably not hurt that much.
 * He left most of his mass in the theater, while only going with a smaller body's worth to the steel mill. Thats why the theater is boarded up - Clayface is still in there.

Webcomics

 * In Holiday Wars, April Fools' Day is a shapeshifter whose limitation is that his mass always stays the same. Labor Day explains this in this strip.
 * Averted in Digger: After turning into a huge monster, Shadowchild points out that in that shape it is stretched out thin and can't do much besides looking scary, and this is with Shadowchild being not exactly solid, but presumably magical, to begin with.
 * El Goonish Shive starts off with a human shapeshifter who can change her density, but not her mass. Then people start getting zapped with ultratech alien devices that don't obey conservation of energy, magic is brought in, and the whole thing breaks down.
 * Felucca's dragon form in Earthsong must be about 10 times her original size. Where does it all come from?
 * Snookums the Tentacle Bunny in The Inexplicable Adventures of Bob was originally a Kaiju-sized giant monster, but he got shrunk to the size of a basketball and is now kept by Molly as a pet. He still retains all his original mass, though, so the earth shakes every time he hops around like a bunny rabbit.
 * In one storyline of Nodwick, the protagonist is forced to drink a magic potion of giant size to fool an invading army of orcs. As it turns out, the potion doesn't increase his mass, only his size, and his increased surface area leads him to be blown away by the ambient breeze.
 * Averted in Drow Tales: Ariel uses whatever she has around to shapeshift into something bigger (such as creating wings), usually her hair and clothes but also a small dragon golem that was created with that ability in mind.
 * Kieri from Slightly Damned switches between being an angel and a snow bunny without much hassle. Considering her clothing pops in and out of existance as well, it's fairly safe to assume that it's guardian magic.
 * Lampshaded in this Tales of the Questor spin-off.
 * The Ambis in Jix all can transform into ugly beast versions of themselves and the more skilled can grow several times larger, such as Kelelder the Planet Thief, his daughter, and Maricax (a bounty hunter). It's never fully explained where the mass comes from or goes when they revert back to their normal size, though the creator hints at Animorphs explanation.

Western Animation

 * In Teen Titans Beast Boy has been everything from an Amoeba to a Diplodocus, yet his base form couldn't weigh much more than 100 pounds. The same likely applies to his comic form.
 * In Transformers, extra mass is stored in pocket universes. Oddly enough, the parts of someone's alternate mode that don't really have anything to do in robot mode don't go away, and instead tend to form decorative "kibble". This technique is used mainly for changing size and storing weapons.
 * The 2007 movie specifically averted this by having the CGI computers track where every nut and bolt went during transformation. The result was that some robot forms were twice as tall as others.
 * Which made it really stand out when the All Spark, a giant cube that had Hoover Dam built around it to hide it, folds up into a much smaller cube that can't be even a hundredth its original size. No one bothers to even try to hand wave it. Maybe it was just lighter than it looked to start with.
 * The All Spark ignores the laws of physics deliberately; it's supposed to represent how amazing and powerful and eldrich it is.
 * The original cartoon did not explain where, for example, Optimus Prime's trailer went when he transformed from truck mode-it's just shown going out of shot or into the frame as required. This has become a long-running joke in the fandom, and even appeared in a Transformers Animated short, where a kid asks Optimus where his trailer goes. As it turns out, Optimus doesn't know himself.
 * There was, in fact, one episode of the G1 cartoon where, as Optimus transformed and his trailer started to move offscreen, it actually glowed briefly, then vanishes.
 * Oh, now I get it.
 * Which becomes funny as in one incarnation, the trailer served as a holder (or whatever) for a pair of secondary robots. In the film series, it actually holds Optimus' spare guns. Lots and lots of spare guns.
 * In the IDW comics (the -tion miniseries at least) mass shifting, as befitting dimensional rupturing, required a lot of energy to do and generated quite spectacular fireworks due to laws of physics being broken. The headache was simultaneously decreased and increased by only having one character consistently mass-shift, but of course it was Megatron turning into a gun..
 * The Chip 'n Dale Rescue Rangers episode "Dale Beside Himself" features the Fleeblebroxians, a roughly mouse-sized alien race that can transform into anything with ease, for example a dragon the size of a small dog and several hundred times the mass of a chipmunk. This can hardly be explained by the fact that they consist of "unstable molecules".
 * Imp from She Ra Princess of Power routinely took on forms that required an outside energy source to perform their function (and more energy than he could reasonably produce naturally) and he was never shown having to recoup what was lost. Notable examples are being a lit candle, two types of rocket complete with jet propulsion, flame thrower powerful enough to start a forest fire, and a laser rifle.
 * Challenge of the Super Friends never explained where Apache Chief got the mass to grow 50 feet tall whenever he said "Inekchok!". People speculated that 500 cattle disappeared from the Great Plains whenever he did this. Clearly, this is ridiculous -- everyone knows that a Native American would absorb 500 buffalo, not 500 cattle!
 * On The Fairly Odd Parents, every fairy suffers from this. Let's just say "A Wizard Did It" and move on.
 * The same fairies that regularly POOF! things into existence? The baggage is one of the least Egregious things about them.
 * Jake from Adventure Time is an extreme example, being able to stretch to Attack of the 50 Foot Whatever-scale on demand. One episode, "The Limit", explored what limit there is to Jake's shapeshifting, and he didn't reach it until he had stretched a great distance. But that was due to his organs no longer functioning, so the hypothetical limit of how massive he can become hasn't been explored yet.
 * Nobody's really sure where Kaeloo gets her extra mass when she transforms. Interestingly, Mr. Cat, the other shapeshifter in the cast, doesn't appear to experience significant changes in mass despite his wide variety of forms.
 * Cat Dog isn't even a shapeshifter, but they can still stretch over miles and miles with no loss of volume when, normally, their body is just a few feet long.
 * Mostly averted in The Spectacular Spider-Man. Sandman looses some of his sand every time he fights, and has to be 'fed' raw silicate to keep the same mass. He only becomes bigger when he ingest more silicate, and becomes a giant after taking an entire beach's sand.
 * Handwaved in Generator Rex with nanobots.

Real Life

 * Plants gain mass via photosynthesis, which basically involves using sunlight to power a chemical conversion of carbon dioxide in the atmosphere into various carbohydrates and oxygen as a waste product. It could be concluded that the atmosphere itself is a plant's Shapeshifter Baggage, containing the majority of the biomass the plant will use to grow, be it a blade of grass or a 100-foot-tall redwood tree.