Superman's physique has been queried for years, as in, why does he need to be that muscular if his strength comes from the sun.
I always thought that the explanation was that that the sun doesn't directly make him powerful; it nourishes his cells, which are able to absorb the solar energy and transform it into something his body can use. Combined with the advanced physiology of a race which is thousands of years more advanced than us (and who likely would have had genetics/science eliminating things like sickness and obesity etc), that energy is intended to make him strong and durable - stronger muscles, better immune system, etc.
I like the idea that Kryptonians were much like humans once, but found a way to alter their genetic makeup so that their bodies could absorb solar energy - less reliance on food and farming, better for space travel, etc. But they had never travelled beyond the red sun of their own solar system. On Earth, with a yellow sun and 30yrs worth of absorption, Superman's body was basically supercharged hence all his powers.