What I’d say is that there is no-one on Earth who believes that fat people were enslaved and brought across an ocean to break their backs on plantations, with the descendants of these slaves still haunted by poverty to this day. There do however exist people who think that fat people are discriminated against, and to this I would say that the kids in order to make fun of that fat person had to be taught that fatness is something to make fun of in the first place, and whoever taught this idea to them had to have a reason to do so, and when the same thing keeps happening again and again, there is probably some sort of systemic cause for it.
My point with this is that acknowledging that people experience discrimination or marginalization on the basis of a specific trait, is not the same as saying that this or that form of discrimination is “the same as” the most infamous form of systemic discrimination: every form of discrimination has a different history, different manifestations, different roles and different causes, and intersects with other forms of discrimination in novel ways.
I’ve never been fat myself, mind you, but my dad was, and after he died far too young when I was just a kid, my mom told me that he died as a result of medical discrimination against fat people. Not that I am a physician myself — and by all means we would both be biased to look for someone to blame — but I still today feel like he would’ve lived much longer if the world were just more accommodating for people of all shapes and sizes.
“Dumbing down” has tended to only cause problems for me, since if I’m too laconic and straightforward about a point, people will let their own biases color how they interpret my words, and their interpretations tend not to be as charitable as I’d like. It really is best nine times out of ten to just say what you’re thinking in as few or as many words as comes naturally, with whichever words feel natural.