A good thing to remember is the LAST place you gain weight, is the FIRST place you will lose it. It's where your body doesn't want to store fat, that is why it waited so long to put any there.
Probably you have less fat cells there genetically or something.
I once knew a girl that didn't like to get under 200 lbs because she lost her boobs. She could lose weight pretty easy too, but I have know her to deliberately gain weight in order to go up a bra size.
I always figured it would be easier to buy a boob job myself, but that's just me.
My thing is my hips, they are HOOOYGE, and I want them to shrink. They prefer to stick around however.