[[quoteright:149:[[TheSimpsons http://static.tvtropes.org/pmwiki/pub/images/peabrain2_117.jpg]]]]
[[caption-width-right:149:[[WesternAnimation This is your brain on animation]].]]

->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."
-->-- ''Literature/HowTheGrinchStoleChristmas'', Creator/DrSeuss

In WesternAnimation, {{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 [[Literature/HowTheGrinchStoleChristmas 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 RealLife would [[YouFailBiologyForever cause serious complications for most of these characters]], so this trope is very often PlayedForLaughs.

That being said, there are numerous examples of this trope being played seriously as well, typically in the MedicalDrama or SoapOpera realm. While cases of super-small organs in those works are still [[YouFailBiologyForever 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 YouFailBiologyForever.

'''Not under any circumstances''' [[IThoughtItMeant to be mistaken for]] TeenyWeenie.

[[folder: Anime and Manga]]
* ''DragonHalf'' has a character undamaged by a sword through the head because his brain is "compact".

[[folder: 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."

* ''TheAmazingColossalMan'': 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 [[YouFailBiologyForever human heart was one giant cell]].
* In ''OsmosisJones'', 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 ''Disney/AladdinTheReturnOfJafar'', Genie tries to get Iago to make Al 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'']\\
'''Genie''': [-An itty bitty one-], but it ''is'' there.

* 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 [[{{Dystopia}} his society]]. This is treated as a birth defect, hopefully curable.
* ''Literature/HowTheGrinchStoleChristmas'': The eponymous Grinch is portrayed as having a heart that is far too small to feel love properly.

[[folder: 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.

[[folder: Newspaper Comics]]
* ''ComicStrip/{{Dilbert}}'' tends to use this, particularly on one book cover.
* ''{{Garfield}}'' likes to use this with dogs, particularly Odie.

[[folder: Western Animation]]
* ''RenAndStimpy'': Stimpy's bean-sized brain [[WhoEvenNeedsABrain 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.
* ''PinkyAndTheBrain'': In the TitleSequence, the two eponymous characters walk behind an X-ray machine. Brain's skull has meshing gears, while Pinky's has a peanut.
* ''TheSimpsons'': 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[[note]]though maybe not as small as the page picture; that'd be unrealistic even for ''The Simpsons'', and it is known that the reason for Homer's stupidity is a crayon lodged in his brain since he was a young boy[[/note]]. This enables him to resist blows to the head more easily than normal people (also helps that he has a significantly thicker than average skull), which had great effect in his brief career as a professional boxer.
*** If a HalloweenEpisode 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."
* ''WesternAnimation/SouthPark'': 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.
* ''WesternAnimation/TheFairlyOddParents'': In one episode Timmy swaps brains with an ordinary dog. His brain is noticably the smaller of the two.
* In ''WesternAnimation/DextersLaboratory'', Dexter decides to put a genius-level brain (...[[NightmareFuel 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 ''[[WesternAnimation/CowAndChicken 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, [[FreakyFridayFlip the doctors get them mixed up]].
* One episode of ''WesternAnimation/SpongebobSquarepants'' 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 {{BugsBunny}} cartoon "Buckaroo Bugs" his 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.

[[folder: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.
* 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, ect.
** 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, its brain shrank. It now resembles two shriveled walnut halves on top of the brain stem.