You gotta work your calves really hard to make a difference, since they are already accustomed to lifting your body weight many times all day long.
Jump rope, a whole frikking lot of it, like 8 sets of three minutes each with 30 seconds break in between, everyday... until it hurts & you still do it everyday and are sore for like two months but you keep doing it everyday anyway and then finally you get kind of used to it.
And if you can, calf raises, with any kind of weight is better, if you can get to a gym is your best option to find a way to load up on safe weight (i recently started to use the Smith Machine to do standing and seated Calf Raises & im still consistently surprised at how much weight i can handle)
I just came across this article & thought of this thread, gonna try to incorporate a lot of this in my calf raises next leg day
The link probably won't work (thx skimble!) but you can easily google the main keywords "t-nation puny calves" :)
I've also read elsewhere, that article I linked mentions it, about how the tissue surrounding the calves is very tough and can limit muscle expansion, and saw a tip to do stretches with your calf on a tennis ball or lacrosse ball, in order to help loosen that tissue around the calf and allow the muscle underneath to get bigger.
Google "lacrosse ball fascia release" or "lacrosse ball calf" and check out some ideas & yt videos.