The point of my post was misunderstood. I build muscle very fast and I am asymmetrical. when I use a heavier hammer, i'm afraid it will be worse. what exercises can I do to balance this? 

Umm, you could exercise your "don't worry about it" option.... I think most everyone I know, body builders or not, are asymetrical.  We all favor one side.  I also put on muscle very quickly (even for a girl) and consequently my right arm just fits differently into clothes than my left.  It is what it is.   When i'm done a day of forging my left arm feels as spent as my right, the left being used throughout for holding things in place, lifting items out of the forge, etc.     If you're really worried about it, the earlier suggestions about switching hammer hands should help.  Unless you're talking about some truly freakish difference, I don't think you/anyone else will notice your dominant arm is a bit bigger except, as I said, maybe when you're pulling on a shirt.