I have been every weight under the sun - from the 3.5 stone anorexic teenager, to the 13 stone mother of four.
I can tell you now, that the 'skinny bitch' comments cut me FAR deeper than any fat cow comments ever did.
Even when I got up to a size 6 and resumed normal life, I still got called 'twiglet', 'stick insect' on a daily basis.
I remained skinny (and I was skinny, a bag of bones) until I had my third child and simultaneously went on meds for epilepsy. At which point I got fat.
I still am fat, at 12 stone, but its not going to stop me eating what I want says the person who has just polished off a bag of Doritos and a double chocolate chip cookie...
I've lost a stone in the last two months - but only because I've moved more. And not through choice! 
If someone calls me far, I can look at my spare tyre s, laugh and say yep, I am, but it beats the Hell out of the alternative, being made to feel like an outcast for being a skinny bitch, being the butt of everyone else's jealous digs when all I wanted was to be a normal weight.
I couldn't give two craps if someone called me fat right now, that isn't hurtful to me, it's true! But I got looked up on fb recently by one of my old school acquaintances, and the first thing they typed was "Hi Twiglet", and it brought all the pain rushing back. No, I didn't reply!
If you are fat, take a minute to think how you are affected by people calling you fat. Well, not many people that are 5 stone overweight like me can brush it off like I can. However, and its a BIG however, not all 'skinny' people can brush off your comments about their weight either. Very few might be like I am about being called fat - which tbh I see personally as FAR more of a compliment than being called skinny - but to the majority it is just as hurtful as someone calling you fat.
Why comment on people's weight anyway? Whether slim or curvy, someone will not be happy with their weight.
Call me fat, lardy, chubba, I don't care. Call me skinny, Twiglet, stick insect, I will be deeply hurt.
(My anorexia was through competing in a martial art based on weight categories and pure and simple food phobias, not 'fat' comments)