Bf is calling me fat...

Ok I’m not mad, I’m just a little frustrated. I don’t think I’m fat, but I guess he thinks I am.... so yesterday we went to the clothing store and he knows that overweight people wear high waisted jeans and he was saying that I need to wear them because I’m big. I’m offended, I would never wear high waisted pants because I’m skinny and I’m in shape. He always jokes around a call me fat like that. How do I nicely ask him to stop?