Hmmm I wonder if you're "fat" or if you just feel that way. Even if you're overweight, it's no excuse for them to tease you. I suspect it was due to your superior academic performance that they targeted you. I believe you when you say you didn't show off: you wouldn't need to ... they'd have realised you were smart. As for them making up that stupid, nasty song ... I am unimpressed. That's cruel beyond words, and I'm appalled by what children can do to each other.
I can appreciate that you felt weak, but you aren't weak. You are strong to have withstood their cruelty, and you're strong by coming here and reaching out to others for support.
That sarcastic way of putting yourself down, well if you're doing it to get in before they do, then yes, that's sad. but it can also show a healthy sense of self-esteem, like saying"I don't take myself too seriously". but, as you said, if it's a defence mechanism, then it is sad that you feel the need to do it, but shows how adaptive you were to have developed that capacity to deflect their cruelty before they even start!
And why should you feel that you should lose weight? Even if you're larger, people come in all shapes and sizes. You sound like a nice, intelligent person, and that's waaaaay more important than your size.
I'm glad you found us.

ps of course you will feel weak when people trigger horrible events that happened to you as a child: you'll feel the same way you did back then ... that's why I suggested you think of a few things about yourself that you like, so when you're feeling weak and vulnerable, you can remind yourself that you are strong, intelligent, sensitive, etc. Reinforce those positive qualities, rather than the ones that you feel people are seeing in you.