Some of my favourites:
"I don't wanna get too big."
"The bigger they are, the smaller their dick."
"Cheaters. They just let the steroids do all the work."
"Damn, that guy is huge!! He must spend 6 hours a day in the gym!!"
"You can't get big without squatting." (Yes, this IS a myth)
"I would never wanna be that big." (Uh huh)
"Will it make me bulky? I don't wanna look like those bodybuilder women." (My sister, when I recommend she does some resistance exercise like pushups, lunges and unweighted squats to tone muscle and add some variety to her exercise program)
I guess not all of those are "myths" but oh well.