Calf growth

Ok, here's the question. My calves are....'interesting'. I have extremely developed calves and they're fairly big. Bout 18'' if memory serves. The problem is, I want them bigger, but it seems that all the workout I do to hit them just seems to get them more defined and not really bigger. If I remember to, I'll take a picture of them. I know some people would already kill to have my calves, but I want to be huge.

I already do 3-4 sets of 20, full stack standing calf raises and 3-4 sets of 20 at 235 on the seated calf raise (all the weight we can get on it). Any suggestions on what I can do to add pure muscle mass, or have I just hit the point where I can't really add any more size to them.