Teaser, that thing about training just the one arm reminds me of a famous bodybuilder who had an obvious asymmetry to his body. Famous but just can't find it, so meh. Anyways, while training one muscle will only stimulate this muscle (either only one arm and not the other, or only the legs and not the arms, i.e. isolation training/motions as opposed to compound training/motions), the same happens without dedicated training already. Just check vasculature, it's a reliable indicator of growth. You'll see that one arm has more than the other even if there's no obvious difference in size.

I've thought about that for years if not decades. My conclusion on this is that merely having one side dominant (right handed or left handed) is in fact training in the same sense as dedicated training with heavy weights and lots of repetition and so forth. When we pick stuff up or do whatever, it's the same motions and the same arm or leg, it has no choice but to grow either bigger or a more extensive vasculature or both. I think this is true most especially during our initial child growth and puberty where the bulk of our growth already occurs.

Also, there's a few disorders where one side or limb grows bigger than the other side/limb. Generally it's called hemihypertrophy or hemihyperplasia. Here it's certainly not about training. So I guess it's possible that the dominant side becomes dominant just cuz it's grown more. I mean, if one arm was already too weak to do stuff, I'd use the other stronger arm to do it, ya?

Let's put all this together and imagine that diet is deficient and there's some training going on and there's a dominant side. Since diet is deficient, something's gonna grow less. Since there's some training, something's gonna grow more. Since there's a dominant side, one side is gonna grow more. The deficiency will exacerbate the difference much in the same way that excess insulin exacerbates fuel partitioning in favor of excess fat accumulation. Fix the deficiency and at least we made sure that nothing's gonna grow less, just like fixing excess insulin we make sure that fuel partitioning no longer favors excess fat accumulation.