You are kind of using an extreme example, but I'll go ahead and take it to its logical conclusion:
Women find rich men attractive. No matter what rating a guy's looks have, if he has money, that number gets modified upwards. If he's a 6 look-wise, the money might get him up to a 9. And a guy who is a 9 is going to have a LOT more options when it comes to women than a 6 will, because more women are attracted to a 9 than a 6. With me so far?
This isn't much different from a girl who is very attractive, vs. a girl who isn't so attractive. A girl who is a 9 is going to attract many more men than a girl who is a 6 (though personality issues can modify this score as well), so she will have more options with men. I think we all understand this.
So, if a man is a 6, maybe he can attract the interest of, let's say, as many as 5 women at any given time. From that pool of 5 women, he has to talk to them, figure out a little bit about them, and pick the best fit for him. That girl may well be plus-sized, but she may have the best personality and the funnest to be around, so he picks her over thinner girls, because overall, she's the best choice.
Now, a man who is a 9 can attract a LOT more women. Perhaps this guy has as many as 100 girls to choose from. He'll quickly eliminate 80% based on a few minutes with them, and take some more time with the rest, until he has between 5 and 10 really good matches to pick from. Since he has been able to pick through so many girls, he's been able to find girls with great personalities and who he is compatible with, and still has several to choose from. Chances are, whoever he chooses is going to be a very good-looking girl, and probably NOT plus-sized.
Why? Simple, he is in high demand, so he can choose the very best girl. It's much like TV show The Bachelor: he can pick from a pool of pretty good candidates. And, yes, of course he's going to do that, and so would you.
A hot, model-quality girl is probably going to be with a good-looking guy, or at least one with wealth, status, or celebrity. And why shouldn't they?
BUT... this really isn't a big deal, because most guys aren't going to become billionaires anyway, so there will always be plenty of guys to go around. It's not like every guy suddenly gets a ton of cash; really only a few do.