Why do people feel the need to comment on the amount of food someone eats?

I went to Shake Shack with a friend and ordered a chicken sandwich with a lettuce wrap instead of a bun, a 6 piece chicken bites, and a small iced tea. It was unplanned that we were going there for lunch so I strategically got that to ensure I had enough calories left over for the rest of the day. When the cashier was ringing me up she laughingly started saying, "wow, are you really going to eat all that? You must be really hungry! Are you going to eat it all?" This genuinely did not hurt my feelings or make me feel any way because ???? It wasn't a lot of food???? I was so confused and just laughed back and said "yeah, I am!" I just don't understand the purpose of her comment? If she caught me on a bad day, it probably would have really discouraged me. If I was ordering a little more this day, then so what? It just got me thinking about how one small comment like this could potentially affect someone on their weight loss journey.