When I was younger I was quite embarrassed about this because there is almost a full cup size difference in mine. Asked my doc about it and she said it is quite common for each breast to be slightly different in size. It affects more than half of all women. She said out of the 50% it affects about 50% of them have half a cup size or more difference.

She also said asymmetry affects men's testicles at about the same rate it affects breasts So we aren't alone lol