That's an interesting question. I don't know the answer, but I don't think you'd do a workout for that. I think you have to fatten it up. But I'm not sure, though. Sorry I can't be more help.
Try using the stair master at a high resistance level. One trainer told me to not use the stair master for TOO long unless I wanted my butt to get big! heheh It won't make it big as in fat, but bigger as in muscular. Needless to say, I quit using it fast!!! LOL
I don't know about bigger but more shapely and rounder is poss. Stairstepper, stairmaster, calf raises, ballet for sure. Walking does help firm up mine, too.
It won't make it big as in fat, but bigger as in muscular. Needless to say, I quit using it fast!!! 
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