Monster Suit

"Pay no attention to The Man Behind the Curtain!"

—The Wizard, The Wizard of Oz

The time has come, and after much journeying and many plot twists, the heroes are finally going to face the Big Bad. Unfortunately, there's a problem: said Big Bad is much stronger than they ever could have imagined. Its power is monstrous, its defenses impenetrable, and no matter what they do, the heroes can't so much as get it to flinch. Despair and woe, the villain has triumphed! His victory is assured!

But wait! All is not lost! Upon closer inspection, the heroes realize that this monster isn't the real Big Bad. The real one is actually inside, pulling the strings. Bonus, he's puny, and could probably be knocked over by a stiff breeze (also, provided the heroes didn't bring one of those along, a big sword). After that, the heroes find themselves faced with the much simpler task of tearing through the faux-Big Bad, reaching the real one, and slicing it to bits. Once again, convenience saves the day!

This trope occurs primarily in Japanese media, for whatever reason, but has been known to crop up in Western fiction from time to time. Similar to Mobile Suit Human, except slightly more dedicated. The faux-Big Bad could also be considered The Dragon after the reveal.

Anime and Manga

The Pretenders in Transformers. There was never really a "reveal" that within the Badass Bludgeon was a puny robot (the audience knew all along—it's a Merchandise-Driven series after all), but even he knew how lame his inner robot was.
Averted twice-over in Inuyasha with Juromaru and Kageromaru. Juromaru was tough, but Kageromaru lived in Juromaru's stomach and was much worse. However, Juromaru was a monster of his own, not just a shell for the other.
- Mouryoumaru is an Inuyasha example that plays the trope straight.
Digimon Adventure had VenomMyotismon.
- And Digimon Frontier had Lucemon. Also, his Dragon monster suit? An actual dragon.
Kind of applies to the Vision of Escaflowne manga, where the Big Bad is actually a little kid that projects a much older, uglier image of himself so that he'll look more formidable.
One Piece has Gecko Moria, who hides inside the giant zombie Oars. Though turns out Moria is just as dangerous as the giant zombie.
Inverted in Neon Genesis Evangelion. The Evas are Eldritch Abominations that wear Humongous Mecha suits, and the suits don't make them more powerful, but rather limit their size and strength to a level at which they can still be controlled.
Envy of Fullmetal Alchemist claims his true form to be a house-sized, multi-legged Eldritch Abomination easily capable of crushing the heroes. However, this is a lie; his true form is weak, newt-like form that can fit in the palm of a person's hand. Ultimately, Ed is dismissive enough of him to let Mei leave with him sealed in a jar.
One of the demons Zatch Bell battles is like this, using its powers of animation to move and fight inside a durable, massive body.

Film

In Team America: World Police, Kim Jong-il turns out to be a cockroach from space.
In The Brotherhood Of The Wolf, the dreaded Beast is revealed to be a trained lion in a Monster Suit. Still dangerous, but more deserving of sympathy, given how it was abused into acting like a monster.
Labyrinth. When the protagonists reach the entrance to the Goblin City, they're confronted by a giant robot-like creature. It turns out that the robot is being controlled by a small goblin inside of it.

Live-Action TV

In the Toku Ninpuu Sentai Hurricaneger, the villain Sargain is a small robotic ant piloting a human-sized robot body
In Angel, the demon known as the Beast in season 4 is revealed to be simply a minion of the real big bad, the demon-hijacked Cordelia.

Tabletop Games

Dungeons and Dragons module WG7 Castle Greyhawk. An iron golem is built like a giant mecha, with many attacks a normal iron golem doesn't have. Its head holds a control room with an orc inside that is controlling it.

Video Games

Yu Yevon from Final Fantasy X is the embodiment of this trope.
Lavos from Chrono Trigger, who has two shells. Of course, the inner forms aren't any weaker the the outer ones...
King Boo from Luigis Mansion would probably fit into this category with his Bowser suit.
The Final Boss of Okami gives Yu Yevon a run for its money: Yami the Dark Lord is actually a harmless baby seal in a jar controlling a gargantuan, mechanical sphere.
Shadow Mitsuo from Persona 4 is a creepy floating baby capable of summoning an "outer shell" in the shape of a giant, pixelated 8-bit warrior. Though the real deal is far more dangerous than the shell.
Poseidon creates one made out of the sea in God of War 3 to fight Kratos and Gaia in the beginning of the game.
In Mario & Luigi: Bowser's Inside Story, the final fight seems to be against Dark Bowser. However, when you pound on him enough, he spits out Dark Fawful which you need to inhale. Inside Bowser's body, Mario and Luigi then need to fight that Dark Fawful. In a slight subversion to this trope, Dark Fawful may be small, but so are the Mario Brothers inside Bowser. And in there, Dark Fawful is everything but puny and in fact very dangerous.

Western Animation

In Teen Titans: Trouble in Tokyo, our heroes are facing a huge ink monster controlled by the Big Bad, but luckily the keystone to the entire thing is the cursed artist whose strings are being pulled by the Big Bad.
Done with a good guy in the Ben 10 movie Secret of the Omnitrix: Azmuth winds up being a Grey Matter in a Cannonbolt suit.
Skulker in Danny Phantom. His real appearance is a small ghost made of ectoplasm, and his normal look is only his Powered Armor.
An episode of Big Guy and Rusty The Boy Robot featured "Po the Destroyer", a giant hulking brute of an alien that mops the floor with Big Guy, but is later revealed to be a very short alien in a robot suit.
Similar to the Danny Phantom above, Se?Siniestro from Manny Riviera: The Adventure of El Tigre, is just classmate of Manny and Frida with a knack for controlling a titanical battle suit which in turn can control an even biggerbattle suit.
Done with a hulking bounty hunter in Star Wars: The Clone Wars who turns out to be a 1 foot tall alien in a big suit of armor. Then subverted where he is still an asskicker outside his costume later on. He built the suit more for looks than anything, as a tiny alien is less impressive to potential employers than a huge, hulking alien of unknown species.