It less to do with fatphobia and more with the fact that it's hard to make this dress look really good even if you're slim. For example:
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This model is very slim, but the dress doesn't quite fit her serious look, the high contrast between her skin and hair, and the hue of her skin. On the other hand:
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Here it looks just perfect. It matches not only her body shape and her colors, but most importantly, her joyful, youthful and dainty demeanor as well.
Meanwhile, on Tess, it looks pretty much like a cooking gas cylinder cover:
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And not only because Tess is fat, but because nothing in her demeanor suggests joy, youthfulness or daintiness, and as a suppah moddle working for so many years in the fashion industry, she should know that how good an outfit looks also depends on the way you carry yourself, the hues and the contrast level in your skin, hair, eyes, etc. I'm not a fashion sperg at all, but even I know that. I guess that really shows how much of a rube she is.
I don't know who the person she is referring to is, but I think someone might've pointed her hypocrisy after posting this: