Hercules wouldn’t have big muscles. You get big muscles by lifting heavy things over and over, which damage the muscles and when they heal they become larger and stronger. For Hercules, who could punch people into space and lift planets there’s literally nothing that would pose his muscles any challenge, so he would never develop big muscles. Now if he had low enough body fat (which was not fashionable in ancient Greece) then he could have well-defined muscles, but he wouldn’t look like Arnold Schwarzenegger.