Men who have more muscle tend to have and maintain higher testosterone levels, and men who have higher testosterone levels tend to have an easier time getting bigger and stronger. Men who increase their testosterone levels- either through training and diet or via artificial means- tend to look more masculine. Put differently, men with more muscle look less like most women, and more like the least androgynous men. This has absolutely nothing to do with culture. There is no human culture where men who are weak are considered manlier while women who are muscular are considered more womanly. The importance of strength varies from society to society (usually in some relationship to available technologies and the kind of work that is required of average people) but strength has been a masculinity quality always and everywhere.
Jack Donovan