Springcoil: i can relate. my gf, who is a bodybuilder tells me my ass is too bubbly compared to the rest of my legs. Personally i don't think it is too big but a little out of proportion compared to quads and hams. I've had it my whole life and i suppose i ought to be happy about it cos like someone else said in this thread, "some guys can't grow it to save themselves..." You a rugby player, me a sprinter and if it's functioning for you then keep doing what you are doing.
thing is, if you want growth in only legs then you'll have to possibly change exercises to not incorporate the ass so much. that's kinda tough if you subscribe to the way most training is recommended here. it kinda leaves partial range movements, leg extensions, leg curls, barbell hack squats, close stance squats to parallel. At least that is what I can think of off the top of my head.