Coworker who comments on what people eat and their bodies

So i’m pretty skinny and have always been in my life. Lately my coworker has been making comments about my eating habits and my body. It’s comments like ”how are you so skinny if you eat a lot?” and I seriously don’t understand what his problem is. At first it was cool since he was curious, but now it’s just f*cking annoying. Like why does he care so much? He has no life. He has also asked me before if I have ever been fat and which I replied no.

Today one of my colleagues were having a leaving party because she is quitting and my coworkers baked a cake and I obviously ate. When we were finished he told my other colleauge in a joking manner ”did you see how much cake she ate?” And i told him what about it? Like I didn’t even think about how big or small it was. I was just focusing on eating and enjoying it.