Yeah, it might be genetics. I had huge calves before I had to work for them. Still, when you work a heavy-bag hard, you will feel muscular sore-ness in the calves, skipping (esp) running, so he probably had to work hard for them as well. I am a big fan of fighters with large legs, like David Tua. I had an inclination the guy with the huge legs was going to win, esp when the guy booted him in the chest and it barely seemed to phase him, haha.
They must also give you good stability when taking a kick like that. You see too many fighters with half developed legs/calves, this is a good example of why that might be a mistake!