...by which, for example, I mean slim guys with fat girls or fat guys with slim girls.
Don't you think this occurs pretty rarely?
I kind of see that it's more common that two partners often have same body-mass proportions....
EDIT: (to avoid misunderstandings)
By same proportion I mean: weight to height (I'm not sure whether this is the same as "body mass index")
That means: a tall and athletic person would have the same proportions as a short and athletic person.
But "tall and thin" + "short and fat" don't have the same proportions.
EDIT2: (On the beauty in the eye of the beholder)
According to many people attractiveness is relative, "in the eye of the beholder", etc. So I assume that they mean "also a fat guy can turn on a slim girl" - which is something I rarely see in reality though.
Could this "beauty in the eye of the beholder" phrase actually mean that if I'm fat, I'm more likely to attract other fat people, whereas if I'm slim, I'm more likely to attract other slim people?