Guys tend to do this in a reassuring way, I think. "I'm really into BBWs!" It's announced as if it's supposed to reassure me that they aren't going to take one look at me naked and run, I think? I don't need to be reassured. Or maybe it's meant to suggest some kink; like to let me know that he wants to feed me cakes and watch me weigh myself? Hey: I don't own a scale. If you care more about my weight than I do, we're going to have a problem.
Don't tell me I'm not fat!
This comes from guys who don't necessarily have a problem with my weight -- they just have a problem with fat people.
When you say, "But you're not fat!" Or worse, "Oh you're so (beautiful, smart, sexy, whatever) for a fat girl…" all I hear is how much you hate fatness.
I am fat. There is no denying that fact. When you tell me I'm not, what you are really saying is that despite my body size, I'm not all those horrible things you tend to associate with fatness. "You're not the kind of lazy, stupid, disgusting, [insert sizeist insult here] slob who I expect fat people to be," is not a compliment.