It is a condition where a small or short man will be generally more agressive towards tall or larger men.

The person with Little Man's Disease is trying to make himself seem stronger or better than the bigger person and is generally hostile or moody towards them.
'That geezer over there must have little man's disease. Look at him trying to start a fight with them to massive guys'.
by Oliver Vantini August 5, 2008
Little Man’s Disease: A disease of short males marked by aggressive antisocial behavior and constant overcompensation resulting in pompous mannerisms such as spitting, twitching, swearing, speaking loudly and tough talk.
The disease only affects teenagers or men who are less than 5’ 6” tall. The severity of this disease is inversely proportional to the height of the sufferer. Other characteristics of this scourge are a very short penis, acne, low I.Q and bad etiquette. The poor bastards usually drive jacked up pick-up trucks lacking mufflers. Often these males are homophobic to the point of insanity because of latent sexual orientation issues.
You must have Little Man’s Disease.
He is such a worthless asshole he must have Little Man's Disease.
Little Man's Disease is an untreatable epidemic in this country.
Some of my best friends have Little Man’s Disease.
by Dr. Vinnie Boombaa April 25, 2007