Pretty much all my gf has to do to get me started is just pop her big butt up in the air a little bit and I lose control. I spend a good chunk of every single day finding ways to squeeze, slap, and stare at that amazing big butt of hers.
To the OP's question, I think the more sex I had over my life the more I started to just love big asses. When I was younger I was mesmerized by big tits, but have since found that anything bigger than a handful doesn't really do much for me. The feeling of riding a fat ass, on the other hand, is like a whole other kind of sex compared to skinny girls.
Just thinking about this is getting me way too turned on. I'm slapping on the handcuffs on her as soon as she gets home and giving her one hell of a pounding.