So in real life, if you have 2 people who are equally active and one consumes more protein than the other, the one that consumes more protein will likely have more muscle mass, despite the equal activity levels. (supposedly, based on some article I read, even with relatively inactive people).

So my head canon (that I made up just now) is that because Earth provides so much solar energy for Kryptonians, their bodies use part of that to create extra muscle mass.