I mean, some people just have fetishes. There's whole porn categories about this so i don't think anyone would get rich just off of that idea.
As far as how, I've spent some time reflecting on similar questions and the way i see it, i think that for some, the excitement comes from feeling like you become the sexual object of desire. Like you are doing her a favor and she is the one getting lucky. And you see it in her attitude, her eagerness, etc. She wants to truly show you her worth because she knows anytime could be the last time. You could wake up from the daze at any time. So i think this is the general idea of why you could enjoy and even get turned on by the idea of fukking a chick from a lower league. The funny thing is that you could actually find an attractive girl who feels like the fat chick. Just like you might get a fat chick who overestimates herself. You gotta be self aware of what feeling you are chasing and then find a girl who's psyche is conducive to that.