Obviously, the reason why one muscle is bigger than other is that the bigger one gets more stimulation and does more job. I have been dealing with it for years.

Even before I started body building as a child, I noticed the right side of my ribs was more developed and more shaped than the left. I even had some peak on the left down on my chest bone, where pec starts. Later, as I started to do some pushups and other stuff (I didn't know anything about muscle gain and body building) I noticed that my left pec was smaller and has a little different shape than the right. I am right handed. My friends told me it is not so visible, and some of them had similar problem.

Later, when I started to deal with bodybuilding, I decided to correct this from the very beginning (maybe it was the reason why I started with weights). Although I am right-handed, I did the exercises where I tried to use left pec more than right (for instance, uneven dumbbells), pushups on one hand (left of course), and others. In several months or maybe a year, my left pec, arm, delt and lat became visibly bigger than the right one, but the pec shape didn't change much, perhaps a bit. But it was interesting that bench press pumped my left pec much more than right.

In a few years, I gained some muscle mass, the ribs were not so uneven any more, the pec on the left of chest bone was not so visible (except for me), but my left side was much stronger!!! Now that has become a problem. I have decided now to try to strengthen the right side of the body to even it. Believe me, I couldn't do that to this day and I am 28 now. My left side is still stronger, although I am right handed. And I think the reason couldn't be in skeletal structure (at least in my case), it's in body position during the exercise. In the days of my beginning of bench-pressing, I was flipping more than 50% of weight to my left side. It was from the very beginning, when I was still learning the move and groove. That was the time when my left side became stronger, and my body became to adjust its position during the exercise.

Let's take bench press as an example. Everyone has a stronger and weaker side of the body (look at the athletes carefully). During work with serious weights, you will do everything to lift that heavy weight and your body adjusts its position in accordance with that, enabling the stronger side to do more. That happens independently from your will every time you work with serious weights. Well, someone will say, use the dumbbells instead of barbell. I tried to do that as well, but still my left pec gets more pumped, not to mention that I could work with more weight if I had 2 left arms. Dumbbells only helped me to make my arms even, and that wasn't big problem, I used 2-4kg heavier dumbbells for weaker arm and that was it. I avoided doing extra reps for weaker arm, you know, the essence of strength and muscle growth isn't in number of reps, it's in intensity and mental ability to concentrate to trained muscle. But my pecs, delts, and lats are still uneven. Although my friends say that it hardly visible, I am still trying to even them.

The thing that gives me best results is when I use lighter weights and concentrate on the right side. For instance, in bench press I use 90kg (I can do 120kgx6reps) and do some 8 reps with deep concentration to activate right pec maximally. It makes my pecs sometimes very even (when pumped after that). But when I do full bench, it is impossible to make them work evenly. So that is my story, I didn't want to bother much.