As someone who was skinny and got fat, here's a secret the world doesn't tell you. The fatter you are, the dumber people assume you are.
My intelligence throughout my time as a skinny person was never questioned. If I was in a place it was because I deserved to be there (sort of, engineering was different) I was asked for help in most classes.
But now? Now that I have a masters degree, am an expert at certain things? People just HAVE to explain very simple concepts to me. Things that I very obviously know about.
I don't look more female now. In fact, people do this even when they mistake me for a man. But I /am/ a fat person now.
The fat idiot is a trope and it's very, very harmful and I wonder if there are people out there that don't know that they are underestimated this way because of their weight because I only know because of the contrast!
As such, I'm very, very sensitive when people explain things to me I already know. I snap very easy. I hate mansplaining doubly much. I just... I can't deal with it.