Your mamas so fat when she went to the top of the world trade center, it collapsed
Do you think you would be offended if somebody said that to you?
I wouldn't mind if someone would make this kind of joke to my mother, she is fat anyways.
There are many reasons why people would be overweight, and making fun of it isn't going to help them lose weight. Making light about it can be hurtful, just like you would be hurt if people made a joke about something that you struggle with.
You are too sensitive.
It's good to be sensitive to other people's feelings. That's how you get along with people better.