I know a guy with crazy big outlandish legs and a medium sized compact butt, but it is only he had absolutely NO butt when he began with heavy squats.... So go figure, you squat the ass will grow, he was lucky in a sense that he needed to build some butt in the first place...
It's mostly genetic , if you are lucky than you have a different fibre makeup between ass and legs so that you can train in a reprange that predoninantly causes hypertrophy in the legs, also if you inherit your mothers genetics, it can happen that you store fat/water round the hips instead of the belly etc.. which will offcouse contribute to ass-size...
Personally I find that SLOW legpresses with a heavy weight for 9 reps mostly terrorises the quads while keeping the tension on the glutes below peak levels. What you lack in brutal intensity compared to a heavy squat, you make up by doing
DOUBLE DROP SETS(!!!) on the leg-press, you need to take 30 seconds between each drop to allow your heart and lungs to recuperate just a little...
Take 2-3 minutes between sets