It's genetics. I have naturally muscular calves, and I recall always having them, but they really grew even more muscular when I took up cycling many years ago. I dont even train calves in the summer months, as cycling keeps them nice and huge. For me, my strongest bodyparts have always been the ones which pump up easily. Shoulders, Legs, Calves, Pecs have always been easy for me to grow, cause they pump quickly without a lot of effort. biceps and triceps and back, are difficult for me to get a good pump, and have been bodyparts I have had to work at quite a bit to get them balanced. I'm convinced the muscles where you get the best bloodflow and pump, are the one's that become your strengths.