Why does he have such powerful muscles when his strength isn’t due to resistance training?
Because nobody would buy his comic book if he looked like a wimp.
If you want to know where the original super powers came from, read Gladiator by Philip Wylie. A professor at a little college in Colorado does an experiment on his pregnant wife and she produces a child who is basically invulnerable, can see long distances, has super strength, has super stamina, can run very fast, and can leap amazingly high. And pretty smart too - he goes to college in what is clearly Princeton.
I’ve seen someone point out that in evolutionary terms, the bit where Superman lets bullets bounce off his chest and ducks when the frustrated crook throws the gun at him actually makes sense. The former is something that one has to learn to recognize as a threat (and which Supes would know is not a threat to him); the latter is a reflexive action below the conscious-thought level (a reflex which Kryptonians in their native environment, being as vulnerable to heavy thrown objects as humans, would naturally develop – Supes could consciously suppress the reflex, but since it makes little difference might not bother to do so).
The difference being that you can’t see bullets in flight. (Well, Superman can, but you know what I mean.)
This version has yet to fight Chuck Norris.
Here’s my question…
What’s different about a red sun versus a yellow one? I mean, light is light, right? Color makes no difference; so his powers would be the same.
See astronomical spectroscopy. Different classes of stars have different light-signatures. Apparently Kryptonians are somehow attuned to absorb the energy from a sun with a yellow-star signature but not one with a red-star signature.
Well, we know which star now: LHS-2520 (aka Rao)
And there’s more than one difference between a medium-sized Class G star (the Sun) and a red dwarf Class M star (Rao).
Post-Crisis, it was explicit that his Greek-god physique was due to his super-powers. The temporarily leggéd Lori Lemaris was in his apartment, working out (and thus showing off her gams) and she commented that, unlike Kal, she had to work out to look good.