When your one arm is bigger, I am quite sure it is because of your triceps and not your biceps. I have a little difference myself as well, though I don't know why. I am just trying to concentrate more on the left and when doing dips and skullcrushers... I take it like I would be doing them one handed and the right justs helps a little bit... sort of. Anyway, the next day my left triceps is sore a bit, right one isn't, and my left arm has almost come up to par.