There was an animated show i watched last summer(I had it playing while i assembled four new detolfs!), and in it, The Flash created a new reality. In any case, in this new reality, Superman was imprisoned when he first landed on Earth and had been kept underground away from sunlight for all these years. He was stooped and skinny, but when he finally got access to sunlight was just as powerful as normal, even though he was not muscled.
For me, this is probably one of the coolest ideas. That Clark could be 5 feet tall and skinny and still able to move the earth and be invulnerable.
Superman's physique has dependent on the artist..I've seen him built like a bodybuilder in one story arc and the size of a normal guy a handful of issues later