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."
In comedic works 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.
- A series of Burger King commercials had two young Middle-Eastern men on a date in a Burger King with two attractive and friendly but bubbleheaded American girls ("So you're from the Middle East? Isn't that like the capital of Arabia or somewhere?"). As one of the girls is listening in awe to her date the camera zooms in to her eye to show what is going on in her brain, which appears to be roughly the size of a pea. The original commercial can be seen here with the next two in the series here and here.
- Dragon Half has a character undamaged by a sword through the head because his brain is "compact".
- 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."
- In Aladdin: The Return of Jafar, Genie tries to get Iago to make Aladdin and Jasmine reconcile; one attempt is startling the parrot resulting in this exchange:
Iago: Jeez! What're you tryin' to do, gimme a heart attack?
Genie: [appears as a doctor] Aha! You do have a heart! [poofs up an X-ray over Iago, revealing a tiny heart in his ribs] An itty bitty one, but it is there.
- 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.
- In Osmosis Jones, the kid who does the project on oyster filtration (which he just started the morning of the science fair) is mentioned by Ms. Boyd as having a brain the size of a tangerine.
- In the seventh Captain Underpants book, when Melvin and Mr. Krupp undergo a "Freaky Friday" Flip, they also get the other's brain. A Funny X-Ray shows Mr. Krupp's body with Melvin's huge brain and Melvin's body with Mr. Krupp's tiny brain.
- 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. His Heel–Face Turn is accompanied by said heart growing three sizes.
- 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.
- One Horrible Histories sketch on Stone Age burial rituals fades out to Rattus and a single tiny pea on a plate: "Here's a brain I've prepared myself. As you can see, from a PE teacher! Hah!"
- Doctor Who. In the prequel to "A Good Man Goes To War", alien blackmarketeer Dorium sells the brain of a Judoon. Despite being humanoid rhinos with large heads, the box containing the brain is quite small; the Judoon being portrayed as not too bright.
- In a Monty Python's Flying Circus sketch a judge has a brain that is so small that it gets stuck in cavities in the inside of the skull, making his speech unintelligible. He has an assistant who knocks hard on the back of his head to dislodge the brain.
- Garfield likes to use this with dogs, particularly Odie.
- Ren & 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 (it also helps that he has a thick layer of fluid between his skull and brain), which has great effect in his brief career as a professional boxer in "The Homer They Fall".
- If a Halloween Episode outtake is to believed, Homer has no brain. A demon went bowling with his head. His head cracked open and inside was a note "I.O.U. One Brain. Signed God."
- South Park: In "Osama Bin Laden Has Farty Pants", the boys go to Afghanistan 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 to be the reason why he blew up the World Trade Center.
- The Fairly OddParents!: In one episode Timmy swaps brains with Vicky's dog Doyle. His brain is noticably the smaller of the two.
- In Dexter's Laboratory, Dexter decides to put a genius-level brain (from where, we don't know) in his sister Dee Dee's head. He needed a pair of tweezers to remove her old one.
- In an episode of I Am Weasel, I.M. Weasel and I.R. 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 Sponge Bob 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.
- In the Bugs Bunny cartoon "Buckaroo Bugs", Bugs' opponent Red Hot Rider (an incredibly stupid cowboy) is implied to have a very small brain. Every time he gives an affirmative nod a small rattling sound is heard inside his head.
- Oh No! It's an Alien Invasion has the Brainlings, a race of aliens with brains in jars on their heads. Most of their brains are small compared to humans', with their leader Emperor Brainlius — the dumbest of them — having the smallest. Brainlius' assistant Briiian has the largest brain — larger than any human's — and he's the smartest.
- In the Codename: Kids Next Door episode "Operation S.P.R.O.U.T." when the gang enter Numbuh Four's body to remove a brussel sprout he'd accidentally eaten that was making him sick, at one point they enter his brain which is the size of a pea.
- In The Venture Bros., this happens because the Ventures' shrinking technology is a little buggy:
- In "Escape to the House of Mummies Part II", Dr. Venture, Pete White, and Billy Quizboy put their heads together to rebuild the dismantled shrink-ray gun, and then they proceed to the testing phase where Billy is selected as the guinea pig. The first test seems to have no effect, but he croaks out that they shrank his lungs. They consider this a blessing, since Dr. Venture just farted.
- In another test, they shrink just his head. Ironically, since Billy suffers from hydrocephalus, this means that we get a brief glimpse of what he would look like if he were normal. But he insists that they fix him, and they get it wrong by enlarging his whole body.
Pete: You look like Toby McGuire in a bad Hulk costume.
- When Sgt. Hatred invades the Venture compound while suicidal, he tries to off himself with a shot to the head. Unfortunately, he uses the same buggy shrink ray, and ends up with "a little baby tongue". This doesn't seem to affect his speech, and presumably it's corrected off-screen.
- One episode has Doc recalling an instance from his Hilariously Abusive Childhood where, at his birthday party, one of his dad's friends pulled down his shorts and shot him in the crotch with a shrink ray.
- In the Donkey Kong Country episode "From Zero To Hero", Cranky Kong has an X-ray machine and is giving everyone free check-ups. When he gets to Funky Kong (Mellow surf-dude to the extreme), he finds that not only is Funky's brain tiny, but also floating in a liquid that fills half the skull.
Funky: How's it look?
Krusha: Serious case of water on the brain.
- 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.
- It is well documented that brain shrinkage occurs with aging. It's part of why the elderly are encouraged to keep their minds active, since as the brain shrinks, dementia can occur.
- Brain shrinkage, or something very much like it, also occurs in football players, boxers, and professional wrestlers who suffer repeated intense concussions.
- When a person lacks the ability to use one of their senses, the brain area controlling that sense shrinks. It's most common with people born with these conditions, since the areas never get a chance to develop as normal. In a blind person, it will be the visual centers, in a deaf person the auditory centers, etc.
- This is also true for babies and toddlers that are not stimulated enough. Neurons don't develop properly and their brain is smaller than average. This usually results in death and/or developmental issues.
- The Koala has a brain like this. Its brain used to fill the whole cranial cavity, but when it adapted to a diet of eucalyptus leaves (which are very low in nutrients), its brain shrank in order to save energy. It now resembles two shriveled walnut halves on top of the brain stem.
- Tree sloths had the same thing occur. Completely unsurprising as koalas and tree sloths both evolved to occupy the same ecological niche.