I know this is kid of personal but i order to help u i need to ask u Which hand do you use when you masturbate?If you use the left hand most likely your problem came because you use\work your left side when you masturbate.I am right handed and i masturbate with my left hand and like you i was wondering why the heck is my left bicep,ticep,trap bigger?Then i talked to a instructor in the local gym about this and he asked me "Do u masturbate with ur left hand ?"He said that this is common among people who are right handed but use there left to tickle the pickle
.So if it relly bothers u shoud try to do it more often whit the right hand.Hope this helps you
.