In the nicest possible way I think you are very confused. Other people have tried to explain this to you, but I will try again, based on this:
"appropriating feminism to a particular issue that is not healthy, or empowering for women."
Fat acceptance is not promoting being fat.
Fat acceptance is promoting not penalising people for being fat.
What IS healthy and empowering for women, is for women not to have their bodily profile at the heart of their persona in the world. For people not to have the right to abuse them or discount them because they do not have patriarchy-sanctioned bodies.
As a separate issue, you believe that the fat itself is a problem for people to have too much of it. I really think that is the bit you need to let go of. Because:
1 - as explained above, that is not what fat acceptance is about
2 - anyway it is not necessarily in itself unhealthy or disempowering
3 - if it were, that would be none of your business
4 - there may, quite literally, be nothing they can do about it. honestly, this is sometimes the case and very little understood by some people,
5 - even if there is something they can do about it, it could quite literally take up nearly everyone's waking hours dealing with the matter. I am not obese (in your sense, or in the actual sense) but I put weight on very easily and sometimes I could cry when I think of the waste of my life spent in calorie counting, agonising about my weight, exercising without enjoying it purely to burn calories, and being non-functional because I am deliberatley operating on a severe calorie deficit. I have wasted my life on the matter. If I had not, I would probably be twice the size, but who knows what else I would have accomplished. If a person has the guts to stand up to society's hatred and qualify as a lawyer and fight human rights abuses (or be a musician or a teacher or a doctor or a bloody good parent or whatever else they could do) rather than fainting on a treadmill for 2 hours every day, and as a result weigh 20 stone, well in my book bloody good luck to them, and they are a better person than me.
so when I say "let go" I mean this very seriously, it is a crie de couer. The pressure of the gaze of everyone who would prefer you to be smaller is leading to the gradual erosion of having the time, inclination or self confidence to lead an authentic life. So please please please let go of other people's fat.
I suspect you imagine that person being just as they are but eating fewer crisps, wearing a 12 / 14 and (you imagine) living longer. It isn't like that. It is giving up your life. For some people they have simply no other way to be and so when you say "you shouldn't be like that" you are saying "I don't approve of your existence"
Keeping people busy on treadmills and hungry is a way of utterly negating their existence even if they don't actually die
of course some die too