Good question. For serious answers, try asking here https://lemmy.world/c/askscience or other similar communities.

On my part, I can vaguely extract some theories from memory.

One suggests some kind of insufficiency or excess during the growth phase can result in abnormal size of one or more organs, whether larger or smaller and usually accompanied by circulatory problems.

Another suggests it a remnant of evolution, whether specific to a type of environment or early societal development.

Of course, there’s also the chemical imbalance issue caused by unchecked industrial exploitation and pollution that causes the entire biological chain to mutate uncontrollably.

But at their base, it’s usually hormones that are directed in some way by something either to grow or to stunt.

Sometimes. Once out of the hatch, environmental factors change each individual potentially in a different way. So while the template might be the same at start, further mutations can branch out in other aspects.

LLMs. Clearly they suck at their job and an AI should take over.

There is another…

How long has it been since you last took a picture of yourself?

When you grow up from a cute little frog to a nasty old toad.