Welcome to the world of being human, bro! haha. Even when you look at the best BBers out there, none of them are exactly the same from left to right. There are a lot of factors that go into this. If you have ever experienced an injury on the smaller side of your body, there is a chance that could be a factor. Could be an injured delt, pect, lat, ect. doesnt have to be the arm itself. If this is the case, you should look into ART therapy and see if there is any adhesions or scar tissue build up. Even if you havent had an injury, ART can help. Sometimes a muscle goes into constant spasam and doesnt flex properly.

Another point is that there is a good chance that it has to do how the muscle is connected. Each muscle has a slightly different insertion and shape. We all have one arm that looks better than the other. I can see a difference from left to right, but when i tape them, they are just abou the ecact same size. Shape has a lot to do with visual illusion. 

If you tape them and one is much smaller than the other, my suggestion would be to focus on unilatteral movements, exercises that allow you to use each arm independently. Over time, this should help to balace you out.