I remember a study from back in college. A couple younger guys that had died in car accidents had both left their bodies to science for study. Just regular guys I believe, not big lifters. The researchers disected both anterior tibs on the guys. In both, their dominant legs had around 20% more fibers than the non-dominant legs.
But then you've got the chicken-and-the-egg thing. Were they born with more fibers in one leg, thus causing that leg to become dominant -or- did they develope more finbers in that leg because it was dominant. More than likely it's the later one, but no way to prove it in humans at the moment.