Al,
I don't think that's it, because I have held the belief that fat girls can be extremely pretty for a long time. However it may just be so unconscious of me to associate fat with ugly that I still get surprised by this.
El D,
Mandy Moore is a great example. This isn't the best picture of her, but it illustrates my point perfectly. You can't tell from this picture how fat she is, but even if she were I would still find her extremely cute.
I think it's in the dimples and the chipmunk cheeks, I guess I am just very attracted to them. So I suppose that if those fat chicks lost weight they would still have cute faces, but they would lose that "fat-chick glow" that many fat girls have, which I believe is also part of their cuteness.