My friends vary in attractiveness. We are all beautiful people, some of us are just less “traditionally” beautiful on the outside and have to rely on inner beauty - while some of us - at least before we hit middle age - stopped traffic.
All the theories noted above have validity. The large women in the OP have food in common, their attractive counterparts have a common interest in grroming.
We tend to be attracted to people who make us feel comfortable, so confident people will find similar friends, where (to use aother example above) a woman with self esteem issues might pick friends whom she perceives to be less attractive, to boost her own ego.
Is anyone else mentally sifting through their friends right now to figure out if you’re a “hottie” group or an “uggo” group? 
For myself, I can just say that the “hotties” and I don’t have much in common. That is, if you’re defining hotties as thin, affluent urban women with lawyer husband types and constantly updated wardrobes. If we meet at a party, we make small talk for a while, they ask me a few questions about herbs for weight loss and then we go our separate ways at the end of the night. It’s nothing to do with their looks, but their reality. I’m much more likely to become friends with the mom of three who shops at Wal-mart and can share some killer crock-pot chicken recipes or a slightly hippiefied type who can keep up with me when I pontificate on the nature of the universe and enjoy a good bonfire.
And when you hang out with people who camp a lot, you hang out with people who don’t rely on a hair dryer or wear makeup everyday. There are some beautiful women who are friends of mine, but they’d need a makeover of more than their hair and makeup to pass as “hotties”.
A couple of things. First is about grooming and style: people tend to hang out with others who share their interests, and some folks love to shop for things like clothes and makeup and all that. I hate to shop, so my bonding time won’t be spent doing that with other people, and as such I wouldn’t grow friendships with people like that. Coincidentally, not constantly shopping leaves me in mail order clothes that aren’t stylish. Same with mani-pedi, expensive haircuts, makeup, etc.
But I will also say that as an “absolutely huge” woman, it is more comfortable being with other larger woman who happen to also share the struggles I do with food, and to a lesser extent clothes, and some of society’s attitudes. They accept me for who I am because they want to be accepted for who they are.
One of my good friends is a hippie momma. Organic vegetarian. Camper. Did RenFest for decades. Liberal as all get out. At least three tattoos and multiple piercings. Married to a guy with hair to his mid back.
Also appears in public most days perfectly made up (in organic cruelty free makeup, I’m sure), buys her designer clothes at consignment shops and has the cutest little soccer mom bob haircut, tattoos covered by hair or clothes. She’s always been hot in the traditional manner as well. I suspect you and she would have LOTS in common.
I’ve been told by outside observers that I have very attractive friends, and before that, it really never occurred to me, but it’s true. That got me thinking basically along the same lines as the OP (because when I made these friends, I was considerably hotter)… I started wondering why I would see groups of hot people, groups of fat people, groups of ugly/awkward-looking people, and so forth.
I came to the conclusion that, at least for me, a lot of it has to do with social skills. I gravitate to people who are confident, comfortable, and socially practiced. Those people tend to be attractive, (which makes sense to me, since more people probably want to talk to them, so they’ve had more experience being social). I can’t stand social awkwardness; it makes me cringe, even in small doses… and not surprisingly, socially awkward people are often that way because they’ve had less experience being social, and this seems to correlate with being less than average on the attractive scale.
I don’t think this is exclusive to women, though. I just think guys are more likely to be confident and comfortable despite their looks, so they end up in groups of more widely varying attractiveness.
I don’t know what the actual statistics are. I just know that my friends, both male and female, vary widely in terms of attractiveness (with me somewhere in the middle), but then I’m the sort of person who mixes with a bunch of different crowds/varying interests/etc. What my friends do NOT vary widely in, however, is character.
I don’t choose my friends based on their attractiveness, but I think because of other factors they all tend to be similarly attractive (educated, from upper-middle-class families, white collar jobs, interested in looking good).
I’m part of a group of three very close-knit friends. You would think we were cobbled together by the same people who constructed the Monkees, or possibly any one of the thousands of young adult “ensemble cast” books. We have three different hair colors; three different body types; short, medium and tall; and three completely different styles when it comes to dress and hair. All three of us qualify as “cute”, but in completely different ways. And interestingly, I seem to be the only one who thinks of myself as attractive.
I have no idea what this says about your question, other than that not all girls choose their friends by attractiveness ratings, or even by other factors that might influence the attractiveness-meter in the same direction.
I’ve always chosen friends and gotten on with other women based on their sense of humour and sense of self-deprecation more than anything. Regardless of how tremendous (or ordinary) they look, or how smart (or dumb) they are, if they are not prepared to take the piss out of themselves and their looks and intelligence, they’re not worth knowing.
That’s how I choose friends anyway, and it’s rarely let me down.