I don't think this is a particularly shallow question -- it IS a legitimate concern in some chapters.
I know that last year before rush, I was frustrated with some body image issues (gaining weight from dorm food, etc.) and rush did NOT make that easier. Of course the anxiety over it is worse than the actual experience, but I didn't look forward to spending two weeks talking to size zero prom queens and ex-cheerleaders when I was already unhappy with my body.
The way I feel is basically that if you've got bad self-esteem, being in a sorority might put you in a lot of situations that emphasize that. If you're anything like most girls, you want to look good for big events -- and being in a sorority means tons of crush parties, formals, mixers, rush parties, and other stuff that you want to look good for. If you're the type of person who's constantly measuring herself against other people and always thinks she comes up short, being involved in a sorority might emphasize this. Some people might blame this on the sorority itself but I don't really think that's the case -- if you have generally decent self-esteem, a sorority is not going to make it bad.
Sororities have a reputation for encouraging eating disorders but again, this is not a problem that only sororities face. The same thing happens in high school cafeterias, ballet and gymnastics studios, and women's dorms around the country . . . Basically, it all depends on what chapter you end up in. Most of the girls in my chapter have a relatively healthy attitude towards eating and exercise -- maybe a little neurotic about the working out at times, but nothing that I would consider unhealthy. One of our past presidents was suffering from anorexia and the sorority was one of the things that actually made her get help for it. On the other hand, I'm sure there are chapters of sororities out there where the girls encourage each other to puke after dinner.
I hate to say "It all depends," but really that is what it comes down to. If you don't have the best self-esteem and you get caught up with a bunch of girls (sorority or not) who are way into the dieting/overexercise/eating disorders, you will probably get into it sooner or later . . . if you are hanging out with girls who are relatively healthy about their body image, you'll probably be "dragged up" to their level (so to speak). Like I said, this isn't a sorority thing per se -- it's something that happens whenever you get a bunch of girls together. My high school cross-country team affected my body image issues more than my sorority ever will, for example.
As for the rest of your post, I'm not going to lie -- I think that a lot of girls that end up joining sororities are girls who were the center of attention in their social circles in high school (not just in terms of looks, but a lot of other things as well). It can be really hard for girls who used to be the "alpha" to be bumped down to beta or gamma
. But this is definitely not the case in all sororities, and even when it is, the benefits usually outweigh the drawbacks -- you just have to keep in mind that YOU are the same girl even though the women surrounding you might be prettier, smarter, more talented, more popular or more charming than the ones that used to be there.