I had a similar thing happen - I went to eat at McAllister's Deli for lunch one day by myself, ordered a large salad, a small bowl of soup and a water. I was seated at a table and was reading a book on the eReader on my cell phone while waiting for the food to be brought out and could hear two women at the table next to me (open dining room, they were on the other side of the center aisle from me) talking about how they both had a hard time losing weight, how hard it was to exercise, etc. They were both what I would consider on the larger size of average, but still average size. I didn't turn and look at them, didn't even acknowledge I heard them (I'm great at eavesdropping without being noticed, comes from being in HR I guess - I always have an ear out for employee grumbling.) The young man who brought my salad and soup had a second plate in his hands - a chocolate lava cake that was about the size of a medium sized dinner plate - that he delivered to the table with the two women next to me. I ate my salad, honestly forgot that they were there - really good book. They got up to leave and one of the two of them stopped next to my table and stared at me until I looked up. She then said "I guess you are looking up the calories in that cake, aren't you? At least we shared it instead of ordering two." She then started telling me that if she looked like me she would be more inclined to eat a soup and a salad but she got depressed so she ate cake. I didn't say a word - just stared at her confused until she muttered something kind of rude under her breath and walked away, and I could hear her friend saying 'some people are just rude' as they walked out.
It sounds like you handled it a lot better than I did, apparently!
I don't know why they approached me - maybe they got defensive since I was sitting there with my phone and assumed I was sending messages about them? Maybe they just noticed the soup and salad and felt weird that it was brought out at the same time as the cake (which I really really wanted, by the way, just didn't get at the last minute)? Who knows.