Shrunken Organ

The Grinch hated Christmas -- the whole Christmas season.
Oh, please don't ask why, no one quite knows the reason.
It could be, perhaps, that his shoes were too tight.
Or maybe his head wasn't screwed on just right.
But I think that the best reason of all

May have been that his heart was two sizes too small."

—How the Grinch Stole Christmas, Dr. Seuss

In Western Animation, Anime, and Comics it is common to sum up behavioral problems or character flaws as being the direct result of some greatly underdeveloped organ in a character's body. Maybe said character has a heart that is too small and thus cannot properly love or maybe their brain resembles the size of a golf ball and thus lacks normal intelligence. Of course, having an organ this small in Real Life would cause serious complications for most of these characters, so this trope is very often Played for Laughs.

That being said, there are numerous examples of this trope being played seriously as well, typically in the Medical Drama or Soap Opera realm. While cases of super-small organs in those works are still factually inaccurate, they are at least portrayed in a semi-realistic sense. For example, this trope could be invoked by a character born with some form of congenital heart defect where their heart is too small and will cause their death unless a replacement is found.

As this trope greatly exaggerates real body parts, it is a natural subtrope of You Fail Biology Forever.

Not under any circumstances to be mistaken for the inversion of a certain trope about that organ.

Examples of Shrunken Organ include:

Anime and Manga

Dragon Half has a character undamaged by a sword through the head because his brain is "compact".

Comic Books

Planetary: In one issue, a giant man is seen in one panel, dying from his sudden artificial growth. His autopsy revealed "a normal-sized brain hanging in a web of nerve tissues like cables in a skull several feet across."

Film

The Amazing Colossal Man: The eponymous character had a heart that didn't grow as fast as the rest of him, which led to him having constant chest pains. A scientist claimed this was because the human heart was one giant cell.

Literature

Inverted in Sonia Levittin's The Cure—the protagonist has a brain that by our standards would be average, but is significantly larger than is normal in his society. This is treated as a birth defect, hopefully curable.
How the Grinch Stole Christmas: The eponymous Grinch is portrayed as having a heart that is far too small to feel love properly.

Live Action TV

Just Say Julie: In one episode, a supermodel's brain exploded out of her head (not fatally) when she tried to think too hard. It was the size of a wadded-up piece of chewing gum.

Newspaper Comics

Dilbert tends to use this, particularly on one book cover.

Western Animation

Ren and Stimpy: Stimpy's bean-sized brain accidentally falls off when he leans down. Ren's cousin Sven marvels at how big it is, and then shows Stimpy his own, pinhead-sized brain.
Pinky and The Brain: In the Title Sequence, the two eponymous characters walk behind an X-ray machine. Brain's skull has meshing gears, while Pinky's has a peanut.
The Simpsons: Mr. Burns' heart has been shown as a shriveled black lump that beats every now and again. Additionally, after crawling out from beneath a landslide, Mr. Burns tilted his head and banged his ear in hope of clearing out the gravel from his other ear. The gravel came flying out that ear, along with his walnut-sized brain.
- Homer apparently has a significantly smaller than average brain. This enables him to resist blows to the head more easily than normal people, which had great effect in his brief career as a professional boxer.
South Park: The boys go to Afganistan and Cartman ends up in a tussle with Osama Bin Laden, during which Cartman pulls Bin Laden's pants down to show...nothing. Cartman pulls out several magnifying glasses until he finally gets enough magnification to show Bin Laden's very tiny penis, which is implied as the reason why he blew up the World Trade Center.
Fairly Oddparents: In one episode Timmy swaps brains with an ordinary dog. His brain is noticably the smaller of the two.
In Dexter's Laboratory, Dexter decides to put a genius-level brain (...we don't know whose) in his sister Dee Dee's head. He needed a pair of tweezers to remove her old one.
In an episode of I.M. Weasel, Weasel and Baboon get in an accident which causes their brains to fall out. Weasel has a huge brain and Baboon has a tiny brain, both disproportionate to their respective head. Understandably, the doctors get them mixed up.
One episode of SpongeBob SquarePants sees Squidward break his "laugh box" (an organ he thought he made up), which had to be removed. The doctor remarks that it's the most dried-up, underused laugh box he's ever seen; it's in a jar the size of a salt shaker. Inversely, Spongebob has a laugh box so big that he was able to donate half of it to Squidward while still retaining the capacity to do some laughing of his own.

Real Life

In a notorious case in France, a mildly-retarded but functional man was discovered to be lacking large areas of his brain, due to a slowly-advancing case of hydrocephalus. Much of what should have been white matter and basal nuclei was nothing but cerebrospinal fluid, yet enough of his cerebral cortex had remained intact to allow his continued awareness and reason.
An undersized amygdala, a part of the brain involved in the experience of fear, is thought to play a role in making a person, not stupid, but sociopathic.
Some people have very small kidneys, or one normal and one undersized. As a healthy set of average-sized kidneys contain about ten times as many nephrons (urine-making tubules) as are required for proper renal function, having small kidneys doesn't necessarily cause any harm.
- That said, there are kidney disorders that can make your kidneys shrivel up and get tiny and nonfunctional; the general response is to leave 'em there as decoration and add a new one to do the actual renal work; once both of the originals have shriveled, there can be more than enough room for a third (or fourth when the third one shrivels) without distending anything.