Are the vast majority of men in relationships with slim women, and are the vast majority of women in relationships with six footers? Would be interesting to know.
No, because as the song goes, 'you can't always get what you want'.
People tend to end up with partners who are similar to them in every way - age, education, income, interests, and yes, physical attractiveness. I know this is MN so people will insist on coming up with anecdotes about how James and Sarah down the road are completely different from one another, but statistically, it is true for most people.
So that means that if you are very physically good-looking - tall, slim, with attractive facial features, chances are you'll end up with someone equally attractive. However, real lookers are a minority and there simply aren't enough of them to go round. So if you're less attractive, you'll probably partner up with someone who's also not a head turner.
Most of us don't get to pick and choose from the most attractive people, but the point is that, if they could choose, most women would go for tall men and most men would go for slim women. Not sure why anyone is arguing to the contrary, because such preferences are very well-established.