I had the same problem a few years ago but it was the left side bigger than than my right side. I started to notice that I always started with my right side when ever I did dumbbell work. I found that I have a boost of energy when you complete the reps you are going for. I suggest what I did was to start with your left first on the first set the your right with your second set and so on. The imbalance will start to work it self out.