From the thefatgirlsguide.com

“When [your friends] say “You’re not fat,” what they really mean is “You’re not a dozen nasty things I associate with the word fat.” The size of your body is not what’s in question; a tape measure or a mirror could solve that dispute. What’s in question is your goodness, your lovability, your intelligence, your kindness, your attractiveness. And your friends, not surprisingly, are inclined to believe you get high marks in all those categories. Ergo, you couldn’t possibly be fat.
But I am. I am cute and healthy and pleasant-smelling (usually) and ambitious and smart and lovable and fun and stylish and friendly and outgoing and categorically not icky. And I am fat — just like I’m also short, also American, also blonde (with a little chemical assistance). It is just one fucking word that describes me, out of hundreds that could. Those three little letters do not actually cancel out all of my good qualities.
[…]
Because fat should mean only having more adipose tissue than the average person, but it doesn’t. And every time you ignore what’s in front of your face to tell me I’m not fat because you can’t bring yourself to put me in that nasty, ugly category, you’re buying in to the idea that real fat people are all sorts of nasty, ugly things I’m not. Horseshit. I am a real fat person, and very few real fat people live up to the worst stereotypes wielded against us.
[…]
Too many women look at me and think, She can’t be fat —she looks fine, then look at themselves and think, I’m so fat — I can’t possibly look (or be) fine. Even ones who are built exactly like me. As long as the horseshit stereotypes persist — that fat women can never be healthy, smart, driven, disciplined, fashionable, attractive and eminently lovable — women who are all those things and fat will keep seeing themselves as fundamentally disgusting and unworthy. So every time someone tries to tell me I’m not fat simply because I don’t fit those stereotypes, I’m gonna keep telling them I am, too, fat, dammit! Le fat, c’est moi. This is what fat looks like.
I am a kindhearted, intelligent, attractive, person, and I am fat. There is no paradox there.”