it's the male reverse of anorexia. a guy looks in the mirror and wants to get bigger because he has a body image problem - thinkin' he is small and weak... as opposed to a girl who wants to become super skinny because she's fat. -- I remember desiring a huge, ripped look like these body builders. then i realized that was not the way to go... thankfully.
It's clearly an image problem because as you pointed out, why would someone want to change their body so drastically...
now, before anyone screams at me, I'm not including people who want to be in shape and have a NATURAL LOOKING physique.