Why is it okay to make fun of a skinny person, but rude to make fun of a fat person?

I am 14 years old, 5'5 and only weigh 85 pounds. I am naturally this skinny. Both of my parents are and I have been my entire life. When people see me, they ume I'm anorexic, sick, don't eat, or have an eating disorder. I actually eat a lot, all three meals and a lot of snacks but no matter what I do, I can't gain weight. People go up to me and tell me, "Oh, you're so skinny". If someone went up to a fat person and told they are so fat people would call them rude and they'd get beat up. They ask me if I'm anorexic or sick. I hear people talking about skinny people all the time and how gross our bodies look. They call me pole, anorexic, bones, lollypop, and tell me I'm long and slinky and make fun of my body and tell me I have no shape. It just hurts my feelings; I cry all the time. It's not my fault that I'm skinny and it's easier and less expensive for a fat person to lose weight than it is for me to gain weight. I have to spend money I don't have to try and put on weight. It's gotten to the point where I have to hide it. I always wear jackets and long sleeve shirts and try to cover my arms b/c I'm ashamed. I only wear long clothing to hide it. We can be teased publicly and told we're skinny but you're mean if you say anything about a fat person. Why is there a double standard?