It’s a question you see a lot — how can someone who is not fat be a good ally to fat people?
There are a few basic things you can do, some easier than others.
First, stop engaging in diet talk, body hatred, food moralizing, and so on. Stop telling fat jokes and equating eating a lot or being lazy with being fat. Even if there are no fat people around. Even if you only mean it in a self-directed way.
Every time you say something negative about bodies, including your own, you are enforcing the hierarchy that makes thin more socially acceptable than fat. So even if you don’t say this in the presence of fat people, the not-fat people who may be around are hearing this and their own prejudices are being legitimized. Additionally, even if someone doesn’t appear fat to you, they may still struggle with body image or an eating disorder, may have fat people in their lives that they don’t want to hear being insulted, and so on. Also, it’s just a rude and nasty way to be and you don’t need that kind of negativity in your life.
This sort of talk includes moralization of food. Referring to things as good or bad, talking about how you need to “work off” something you ate, or discussing whether someone “needs to” or “should” eat something all enforce the same fatphobia. People do all of these things because of a fear of becoming fat. They may claim it’s because of “health” but if that were the case there would be no talk of working off food, there would be talk of vitamin content or eating for joy (because mental health is health, too!).
Stop using the term “flattering” when talking about clothes. That word just means making someone’s body appear to conform to ideals (meaning, making it look smaller). If someone is wearing something that isn’t “flattering,” that’s OK! They probably want to be wearing it, and it probably makes them happy, which is what matters. Or maybe it’s all they can afford, so laughing at them makes you twice the jerk.
Saying “get fat” when you mean “eat lots of delicious foods” is harmful as well. It reinforces stereotypes. People of all sizes have lazy days or go to events where they eat a ton of rich dessert.
Second, speak up when you hear others doing these things. Tell people you don’t want to hear about diets and that what someone else eats is their own business. Tell people that it’s not cool to make fun of someone’s clothes, even if they are wearing something that’s not normally shown on their body type (fat girls in mini skirts and crop tops? Rock on). Point out that eating a lot doesn’t equal getting fat, and that it’s not OK to make that false equivalence. Don’t let people get away with casually reinforcing stereotypes.
Third, speak up when you see a fat person being bullied. Confront people who are making fun of someone and tell them it’s not cool. Obviously, be mindful of safety, but if something feels unsafe to you imagine what it must feel like for the victim. If you can’t directly intervene, go up to the victim and ask if they’re OK.
(If they get mad and brush you off, or even if they are outright rude to you, don’t retort. They’re not actually angry with you, they’re mad at the situation and probably extremely embarrassed.)
If you CAN intervene, shame the bully as strongly as possible, but do so without relying on stereotypes. Things like “that’s not cool” or “her/his body/health/outfit/meal/etc is none of your business” or simply “leave him/her alone” are fine. If in public, don’t be afraid to be loud (unless it would make things worse for the victim).
Bullies don’t react well to being embarrassed or to having the tables turned on them, but they’re also cowards. Most of the time they’ll slink away. When I loudly shamed the douche on the subway who told me to go running every morning, it didn’t take much to get him to slink away. Once attention was turning to him, and people were giving HIM the stink eye, he lost his power.
Unfortunately, we live in a world where members of the socially dominant group’s opinions are valued more than members of the oppressed group. This means if you are privileged, it’s your duty to speak up. Thin people need to call out fat-shaming, similar to how white people should call out racism, straight people should call out homophobia, men should call out sexism, and so on. It will mean so much to the victim, and it makes the world just a tiny bit more pleasant.