Genetics
Well it sounds like your just big a genetically big person. If you want to build a bodybuilder the task will be to bring everything into symmetry. Go light with high reps on your ass and any other part that seems out of porportion. Try to bring everything else into semmetry with your...
Big butts
Take a look at your squating and deadlift stance. This could have alot to do with it. Do not stand wide. Wider stance will give you a wider ass. If you want a nice square ass all pressing should be done narrow. Straight out of the mouth of a former Mr Olympia.