I love this question

So what would be good here is to address the underlying sentiment- that fatphobia is hard and it would be easier to be thinner

To in essence say- I see you and the way the world treats you is not fair.

So let's work out a script for reframing https://twitter.com/Juzams2Plwshare/status/1305625419299745793

Step 1- Acknowledge but without buying into fatphobia and diet culture

(I'm gonna steer you away from using the word fat even tho it's a great word b/c it does upset some people)

So how about-
"I have been reading about how size is used as a way to treat bigger people badly and I want that to change"

Or
"I hate that people are treated badly for being bigger, it's not right"

Then, because I am a nerd, I would probably drop in a little factoid like--

"Hey did you know BMI isn't even a real thing? I definitely did not."

And then-

Hard subject change.

How's your job/kids/cat etc?

What I definitely definitely do not want to see from thin people anymore is denying that fat people are treated badly or anything that contradicts fat people's experiences

Better to acknowledge that fatphobia exists and express that you don't agree with it

And then if the person persists in negative talking themselves I think you can borrow a page from the Good Vibes Only crowd and say--

Oh I don't want to talk in a negative way

And then just go with another subject change

And then if they keep doing it, leave the conversation

But I really encourage you to first respond to what the fat person is probably expressing about fatphobia

Which is that it is hard and it sucks and that we can see how much easier life is for thin people and that is hard to watch

Because we just don't see very many thin people who are willing to go there