Genetically I'd say that I have good girth on my calves. In the last month though I have gone from 16.75in calves to 17.5inch. On seated calf raise you're working your soleus, the soleus supposedly has more slow twitch fibers so go slow. I usually unload the weight that guys with baby calves do crazy fast, and just use a 45 and 25 or 35 pound plate and go about an 8:2:8 second tempo for as many reps as I can(personally somewhere around 12-15reps). On a straight leg to calf raise exercise like a standing calf(I never do) or donkey calf(my favorite) go light but high reps, 30-50 reps for 4 sets pushing perfect form is what I do.