Aggression or needless bumptiousness displayed by a man below average height when it is perceived to be an attempt to compensate for his small size.
A shorter guy is constantly jostled in a crowd by an unthinking taller guy who sees over his head. Eventually, the shorter guy reacts aggressively by verbally abusing the taller guy. The taller guy is superciliously amused because the amount of aggression shown by the shorter guy is disproportionate to his presumed physical ability to carry through his threats. "Oh - it's short man syndrome".