I am not symmetrical either, one arm is bigger than the other, same with quads, tris have different shapes, etc... Best you can do is to focus on evening it out by giving the weaker areas a little more work, and it will get better over time. And the bigger you get, the less noticeable it is, ie, if your arms are a 1/2" different in size, you will notice that much more on a 14" as opposed to a 19" arm