I'm right handed and my left arm is a tad bit bigger than my left. Not enough to be noticed by anyone, but I can tell the difference looking down at them closely. Plus the definition in the muscles is different in each a slight bit. I always use BB movements as the foundation for all my workouts, and then switch to DB or single arm movements as finishers. I've found that when I get to my finishers on things like side delt extensions with a cable, my left arm can actually do a few more reps than my right. This is also the case with forearms. When I finish my back and bi workout with some DB forearm work... the right forearm can't do as many reps and starts burning alot faster than the left does, although the right is bigger and stronger to begin with. I think this is because during the BB movements, when both arms are being used, my body forces the right arm to do more work because it's stronger, therefore it ends up getting more fatigued. This bothered me to realize, because if you allow it to continue to work harder, it's just gonna continue to outgrow the weaker arm. So now what I do, is make sure to add more reps to the left arm on all finishing exercises. Say on dumbell forearm curls, I'll get about 10 reps with the right hand... and it'll be on fire. Well, when I get to 10 reps on the left, it's not hard yet.... so I'll do 15-20 in an attempt to get leave it as worked by the end of the day as the right arm. I've started doing the same on the delt stuff. Haven't been doing it long, but it seems to be working, so I'm convinced the idea is solid. Give it a try, hope it helps.