Weight is a really sensitive topic. People are made to feel terrible about their weight, and treated as disgusting for being “too fat”. The line of “too fat” is pretty arbitrary.
Most people raised as female have been on diets, and most have been pressured into going on diets, and most women are socialized to feel ashamed of eating delicious food that contains substantial amounts of calories. A lot of people are afraid of being fat or perceived as fat, and are worried that they’re too fat to be allowed.
For these reasons, it’s really easy to hurt people by seeming to endorse the position that they’re too fat to be allowed. It’s possible to do this without in any way intending to, and it’s important to learn how to avoid doing this.
Do not express an opinion on someone’s weight, even if they bring it up or are apparently inviting comments. For instance, people often say “I’m so fat” or “I shouldn’t eat this cookie, I’m too fat already”, or “Do I look fat in this?”. People who say this almost never want an actual reply to this, and giving one is likely to cause serious offense. The best course of action is to change the subject.
Similarly, if someone appears to have lost weight, do not point this out to them. Complimenting someone on weight loss almost always suggests that they used to be unacceptable before they lost weight, but now they have become thin enough to be allowed.
(Also, sometimes people lose weight because they are sick, having trouble eating, or feel unworthy of eating – being in this situation and having everyone tell you how great you look is *really* unpleasant).
Some people are exceptions to this and can talk about fat or their bodies without taking or giving offense. Do not assume that someone is an exception to this unless you know them *really* well and have a *specific* reason to think that they are comfortable talking about weight. A specific reason is something like they’ve talked about weight in a neutral way and have explicitly said it’s ok to talk about and that they’re not ashamed of it. Considering someone to be an exceptionally rational person is *not* sufficient evidence to assume that it’s possible to talk to them about weight without hurting them.