Way to not understand the problem at all
The problem is with your BFF appropriating a label used for people oppressed by thin privilege. If you’re not fat and don’t experience fat oppression, DO NOT call yourself fat.
Also, her calling herself fat HURTS fat people. Example here.
And I say, if she was once what some would consider “fat” but is no longer yet STILL struggles with what we fat people do (something that is extremely common) that the best thing we can do is give her a thumbs up for BEING HAPPY IN THE BODY SHE HAS.
I get that her calling herself fat may hurt some people, but then again, knowing where she’s coming from and the context in which she’s saying it, I find myself, if anything, being a little bit more accepting of my size. Perhaps not in this particular context with this example, but overall, when I see someone who has struggled and still doesn’t “fit” the modern idea of “skinny” and still celebrating her/his body? It makes it that much easier for me to look in the mirror and think, “Hey, I like what I see!” I don’t care if you’re a size 12 US or 14 UK, if you FEEL fat but can accept it? More power to you!
I also think dismissing this girl, and others like her, is no better in some ways than what some (and in my experience, ONLY some) thin people do to us big people. We look at them and think, “Oh, hey, they don’t fit MY stereotype, therefore, they are appropriating!” Bullshit.
I personally am a little tired of this “SKINNY PEOPLE (and by skinny I mean those not as big as me and therefore not FAT IN MY OPINION) NEED NOT APPLY.” And I think the braver and more open-minded thing would be to say, hey, welcome, we love you, you’re one of us, pull up a chair and have a cookie or a non-fat-vegan wrap, whatever you should so choose.
And I’m also going to say, and I’m sure I’ll get flack for this if nothing else (though consider I’m mostly ignored around here, I’m only barely worried), there is something to the idea of changing your viewpoint and adapting to new things. Perhaps to us, she isn’t “fat.” But in society at large, and compared to what women in general in the Western world are taught from nearly day one by society, she IS big. Technically, from what I understand, she’s about “average,” but considering the disdain put on what is supposedly “average,” I think it’s being compounded by a bunch of people who she and others like her thought they could relate to dumping all over them.
So instead of just getting it from the “thin” portion of society, we’re getting it from each other? There just seems something incongruous in that. In the end, like the person who was hurt by the OP’s comments on her “chunky thighs” being hurt by that statement, the OP might in turn feel hurt by a few people here essentially ganging up on her and basically saying, “You don’t belong!” How fucked up is that??
More words words words, important words, I must collect them.
Sorry you guys, this is just super fascinating to me but I can no longer form my thoughts so I’m just pushing discussion along. Yeah!

#LasMujeresRealesTienenCurvas: #RealWomenHaveCurves:
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