Good point. He's so much stronger than human that he could be built like Woody Allen and still be the strongest guy in the world. And really, how often would he be able to exert his muscles sufficiently to build them up in the first place? Also, more generally, the modern trend in comics to base superhero physiques on bodybuilders rather than athletes is deeply unrealistic, since bodybuilder physique is exaggerated for display. People who actually need to use their muscles in a practical way aren't so overbuilt; in fact, they're often quite wiry.