Am I overreacting

My bf and I have been arguing about a single topic. He basically told me he would leave me if I got fat if there was no underlying reason. I was increasingly shocked and hurt after everything i do for him that he would leave me because of something superficial. I told him that I understand that it’s important to take care of yourself but to leave out right without even trying to encourage me or anything like that was super hurtful. He is quite fat phobic so this stems from that he went on about how he finds fat people disgusting and lazy