Well now that depends on which version you're going for. It wasn't until John Byrne's re-vamping of the whole Superman legend back in the 80's ("The Man Of Steel" Mini-Series) that they decided Clark Kent/Superman should even *have* Muscles! Before then, he had always been an average looking person. This made it much easier for him to blend in with the regular humans without suspiscion. Heck, that's why no one thought Superman and Clark Kent were the same person... he was just so average! He could have been Clark, or any one of a thousand different guys around Metropolis!