As someone who's been both unhealthily overweight (300 lbs), and unhealthily underweight (175), and STILL was "too heavy", and considered "overweight" by the BMI chart, this whole discussion feels like it's mostly about what it's ok to shame people about.
The BMI crap can go take a long walk off a short bridge. It completely ignores the variety of shapes sizes and characteristics of human diversity. Like bone density, skeletal structure, muscle mass, and so on. My lowest possible healthy weight is 180. At 180 lbs you can already see my ribs and my hip bones. Any lower than 180 and I start to get neuropathy in my legs, random dizzy spells, and other health problems. But BMI says at 180, I'm "overweight". when I weighted 225, I wasn't skeletal like I am now, looked normal, and was perfectly healthy. But MBI says that 225 makes me "obese". BMI can GTFOOH.
I hear a lot of people making it about "health". Cool. But I don't hear anyone talking about making the food available healthier. Or making healthy food less expensive. Or health benefits to help fight obesity. Really, it seems like they just want to be allowed to shame fat people for being fat. Cos plenty of people who "look overweight", ARE perfectly heathy.
If you have health problems, you have health problems. If you don't have health problems, you don't have health problems. Regardless of someone's weight. It's interesting to see people so very interested in the health of others, but only so far as it allows them to give those people a hard time. Oh, and if you try to point out something unhealthy THEY do, you apparently need to "mind your own business". Funny that.
The point of "fat acceptance" isn't that people think "fat is GREAT!, you SHOULD be fat!" - it's that maybe, despite how we may feel about it, this person still deserves to be treated like a human being, and if we really care about their well-being, there are probably better ways to help than by ridiculing them. That's like saying the best way to motivate people is by constantly shaming them for their laziness until they stop being lazy. Does that mean the cure for autism is to shame autistics into not being so autistic?
What this boils down to is the idea that "you look a certain way, therefore it's ok to make certain assumptions about you, and act negatively towards them, because I'm helping!" - that is "you LOOK to be what I PERONSALLY consider to be 'overweight', and that MIGHT be 'unhealthy', therefore I'm doing GOOD by criticizing your weight in the hopes that it coerces you into losing it - for your own good, of course".
It reminds me of when guys tell girls they should "smile more". Despite all the explanations of what "they meant", it still just comes of as "be better looking scenery for me".