My chest muscle, one is bigger than the other when i work out. what can i do about this?

You may have a congenital absence of all or part of the pectoralis major muscle. While not exactly common, it is not rare, either. I think the right side is absent more frequently than the left, but am not positive of this. The defect may occur alone, or it may be associated with other structures such as the hand, the collar bone, the shoulder blade, or even the kidneys.  more

Everyone has one side of the body bigger than the other in at least one spot. My left bicep for example is 1/4 inch bigger than my right. Even pro bodybuilders have this problem. It's only a problem if it is visible from far away. You can work on compensating the smaller size by isolating that side if it is really a big concern for you (for example on a hammer strength bench press machine).  more