Is the shape of the muscle different? Have you injured one or the other? Did you ever play a one-side dominant sport (ie - baseball)?
May help answer the question. You could always do a couple of extra sets of DB curls with the smaller size.
My right shoulder is jacked - injured - and I played in high school, college, then semi-pro for a few years and my right biceps peak is not as good as my left (I through right handed) and I know it's from the injury, nerve damage, and also the constant elongation from throwing. Will probably never be completely symmetrical.