Why do people think it's ok

To start off yes I'm tiny not by choice. My weight was never an issue for me back in high school when I weighted my most and looked really good. Near my end of high school I had alot of family problems going on and eventually when into a depression because of it. Not trying to I lost a bit of weight and have been underweight since which now has been 8 years. I have tried everything to get it back and while I know im thin i have struggled for years to gain it back because i am underweight but I have people telling me to eat a burger acting like I never eat. It's annoying and honestly hurts me because I eat alot I can eat all day and not gain anything and I try so hard. I get people telling me im anorexic or bullemic because of how I look and its not by choice at all. My SO's uncle who is very nice and had never been mean to me is one of these people telling me i need to eat more. I have told him several time I eat alot and he has even asked my SO and my SO has told him its all i do most of the time because i get hungry all the time and will eat about 4-6 times a day with snacks inbetween. He still makes these comments and idk how to handle it without blowing up because its such a sore topic for me and I get it all the time. Idk what to do to handle it because It really does get to me to the point i just smile and nod and leave the room because i want to explode on people who say this to me. Wth am I to do when this happens because i cant take it anymore?