I have been trying very hard to create well rounded, and developed characters in my book. I have them all figured out, except, when looking back, I realized that they are all very skinny. I have a a black, female, main character, a racially ambiguous (although light skinned), female, main character, and a white, female main character. Now, the second two have major self image problems and I was thinking that I didn’t want to make them over-weight, because that may suggest that you have to have self image problems if you are over-weight. But, the other main character is black, and I don’t want to follow that stereotype either. This means that I have a choice, go along with a stereotype, or not provide proper representation of a large group of people.
A while ago, I asked a similar question about representation of national groups. I invite you to take a look at it here. The answers I got boiled down to this: you cannot possibly have every human group represented. Your cast is diverse, that's great. That's enough. Make sure your characters are well-rounded and interesting.
As for stereotypes, if your character is nothing but the stereotype, that's a problem. If your character is the only representative of a group, and follows the stereotypes for their group, especially if they're negative stereotypes, it's suspect. For example, if you've got one Jew in your story, and you've made him stingy, cowardly and abusing children (I'm referring to Fagin), that's bad. But if you've got three Jews, and one of them happens to be thrifty, that's perfectly fine and realistic. Especially if he also happens to have other character traits. If all of them are in fact exceptionally generous, you are calling attention to the stereotype.
What I'm trying to say is, your story is fine, relax. If you're feeling uncomfortable, you can add a side character. Maybe an overweight girl who has no body image problems, like Netta Barzilai. But if you're forcing that character on the plot, then it's better not to have them. A token character, who serves no purpose other than "representing", sticks out like a sore thumb. Me, I'd rather see the groups I belong to not represented at all in a particular work, then forcefully shoved into a story where there's no place for them.
The Story Comes First.
I think you are worrying too much about trivial points, and this can only make your story more difficult. When you want to portray a stereotype without seeming to do that, you need to get creative: Give a reason for it that makes it seem other than the shallow reason: One of my female characters, a spy, must adhere to stereotypical beauty standards because a large part of her job demands seduction and flirting to get her way. I could have a female character that is an actress or on-screen personality; a news anchor or reporter, and this is part of her job too, even if she thinks it is shallow.
To me, the real stereotype is a woman starving herself to be thin to make herself a hottie, sexually attractive to men; thus voluntarily occupying the role of sex object. Even if she does this to attract a mate, she creates the problem of attracting a mate that cares more about her physical appearance than anything else, which may result in divorce when age finally wins and she cannot compete (on the beauty front) with 24 year olds. (We see this "romantic love interest" actresses all the time; once they begin that role they tend to become typecast with relatively short film careers of ten or fifteen years).
But you can break that stereotype by giving your characters a good reason to be "skinny" (I'd call it "thin"), and portray that in a few sentences or minor part of a scene or conversation.
For example, they can be happy and proud to be thin, because it is healthier than being overweight.
Even if a woman has other self-image problems or doesn't think she is attractive, she can be proud she has not succumbed to the temptations of "eating her sorrow" on top of those problems. She can believe being overweight causes a cascade of problems in older age, from difficulty exercising and working out, to diabetes, heart disease, and cancer.
She and her friends can freely admit they adhere to this stereotype because it is just good health sense to be thin. They aren't starving themselves to please shallow men, they just enjoy the activities being fit allows them: walking, running, hiking, some sports like tennis or racquetball or baseball or mountain climbing.
When this is a lifestyle, they aren't worrying about how they look to others; they are worrying about how they feel (healthy, fast, strong, active) and managing their lifestyle to keep that feeling, in part by controlling their eating, in part by play and working out.
Now you don't have to (and shouldn't) info-dump this attitude like I did; it can come out in snippets over time, in conversations or thoughts or prose.
"God," Jackie said, "That was delicious. I could eat three of them."
"Feel free," Karen said. "I'm buying."
"Right, at the end of that road is diabetes, heart disease and an amputated foot, like my aunt Sheila. I'm over it. But baby, those are good."
"I was just being evil. We have to get to the bank anyway, you ready?"
Unless the point of your story is to discuss sexual and ethnic groups, I'd say you're way overthinking this. The purpose of a story should be to make an interesting or entertaining story, not to create a catalog of every ethnic, religious, and sexual group on the planet.
Personally, I just find it laughable that every advertisement these days seems to just have to have a white person, a black person, and an Asian person; and at least one man and one woman. I get it, they're trying to be diverse. But by overdoing it like that you make it a joke instead of a social statement.
Create the characters you need to make your story work. Let their "group identities" fall where they may.