I guess people get mad when you say it’s no big deal to be fat because being less fat than other people might be the one and only thing they have in their entire life to pretend they can feel good about. They need to believe weight is a measure of worth because they just have absolutely nothing else going for them.
This applies to almost anything honestly. Anyone who looks down on other people for weight, sex life, money, physical strength, education or any other empty superficial quality is probably someone who lacks any actual positive qualities. It’s a single step up (just barely) from thinking you’re superior to other people because of the brand of clothes you wear or something.