Obviously genetics plays a role like in everything else, but I definitely think there's something to learn here with the way some of these fighters train... all that volume, high reps seems to work the calves pretty damn good, at least maybe over a longer time period. Skipping rope is one thing, another is the actual technique of how they kick, they go up to their toes when kicking...
So anyone with stubborn calves should try to think differently.
I'm impressed, they've got damn big calves for their weightclass, and they're pretty much in that shape year round.