This disorder is typically more common in males. It is caused by the short person's feelings that society looks down on them for being short. Characteristics include, in any combination, talking loud, bringing other people down, excessive pride/cockiness, and sometimes (my personal favorite) climbing on tall objects, all in a futile attempt to feel bigger. Utterly hilarious to the average-sized onlooker.
Closely related to fat people syndrome
John: OMFG I AM LIKE THE COOLEST KID EVER!!!
Susan: (to Brian) Wow he is that kid?
Brian: Haha that's John. He suffers from short people syndrome
and has to talk loud and cocky to feel bigger.
Susan: Oh my gosh wow look at him now he's climbing that tree hahahaha