The kind of work they take is much different than for other muscles.
My uncle is an avid hiker and his calves are I shit you not almost as big as his (relatively unimpressive) thighs. Its ridiculous really. But similarly, I always notice that hikers, men and women alike, have very well developed calves in terms of size and definition in comparison to the rest of their bodies. I don't know how others train calves, but stimulating them for several consecutive hours walking uphill will definitely make them grow