Seriously, we need pics of aforementioned butt. LOL
(That sounded soooooooooo guy-like)
As your muscles grow your clothes will get tighter all over, in the arms, the back - heck I can't tell you I STILL have trouble with cloting as my arms are still bigger than those who've never lifted and my lats are simply hyuge. It would stand to reason that your trousers will get tighter in the thighs, the seat, but the waist? That shouldn't be happening unless you are doing movements designed to make your waste get thicker....