Genetics, and a boatload of gear.
I'm tired of these guys coming out of the woodwork, looking like Frank Zane +30 pounds.
I would say that work ethic has a lot to do with it, but then you hear stories of guys who are lazy in the gym, and cheat fairly regularly on their diets.
All things being equal, it's genetics, and response to drugs.
Anyone, can become a monster, if they train hard enough, eat enough, and blast enough gear, to give an ox an orgasm.
It's all about how you look while doing it.