Even before I began to lose weight and exercise, I noticed that my right upper arm (my dominant hand) was slightly bigger than my left. As I lost fat and gained some muscle, it seems that the left underarm has tightened up much more than the right has. I definitely have more of the dreaded "batwing" effect on the right arm, and I'm uncertain what, if anything, can be done about it. Any thoughts, oh wise ones? I use hand weights and resistance bands.

Thanks!

One thing you could try is to start using your left hand/arm more for day to day stuff, like when you're carrying things, use the left, etc to try to equal the use they get.