You asked:

"Why do some many people hate fat people, especially fat women? It's mean and not O.K to make fun of us for being big. Not everyone can be the perfect size no matter how much they diet and work out."

Somehow or other these people have gotten the idea that it is okay to discharge their negative emotion on to people who are unable to effectively fight back. Society reinforces them with all of its attention on impossible ideals : women are supposed to be perfect, forever young, incredibly tiny (even immediately after having given birth).

You need kinder friends. If you want to remain friends with people who make hurtful comments about your weight, I suggest you calmly reply: "Your comments are hurtful and unwelcome." Then change the subject. You don't need to explain, justify, or apologize for how much you weigh. You are a wonderful individual, completely unique as you are, right now, this very minute.