Excellent posts, completely agreed. That is exactly what I was getting at. Physiques and bodybuilding are all about illusions. The where and the appearance of the muscle is often as important as how much of it there is. Cavill's crazy combination of small bone structure (see how slim his joints like his wrists are) and wide frame is freakish awesome and the perfect raw material for a great physique.
Based on the strength gains that Mark Twight published and his anecdotes about how hard Cavill worked in the gym, Cavill deserves a lot of credit for how his physique turned out in terms of work ethic and determination. However, I think you need to mainly acknowledge Mark Twight and GymJones for how Cavill looks. He is the one who designed the program to bring out Cavill's genetic potential and used his vast knowledge to keep him on track through tough workouts and guard against injury. Also, as you mentioned Cavill and the Spartans in 300 are perhaps the two of most noteworthy physique to catch the public's attention. I think it must be remembered that the common element between them was not really Snyder, but Mark Twight who was hired by Snyder to make it happen both times. Dude deserves mad credit.

t:
Yeah, I can definitely see the leg mass being part of it, but I think it is equally what slumkat and I are discussing. However, your points about leg mass bring me to another thing about Cavill's physique that makes him look so Superman-ish. He has a relatively short torso and longer legs (part of why they look slimmer and are harder to build as you mentioned). The longer legs make him look taller while his shorter torso exaggerates V-shape silhouette of his upper body. Perfect for making Superman look thick and wide while also tall and not stocky (as Bale was criticized for being in Begins).