Boyfriend calls you fat, just a nickname or nickname. It shows that your boyfriend loves you too much. Calling you little fat makes your heart warm, which is many times stronger than calling you little skinny.
Girls certainly feel fat, even if they are thin, they feel fat, so they are willing to let their boyfriends say that they are thin. Others say that you are not fat, and it is a good sign for your boyfriend to say that you are fat. Enjoy it.