I have a super-skinny friend who smokes. She is confused as to how people get really big--but not judgmental. As a former smoker, I was able to draw a comparison--the craving she felt when she needed a cigarette is very similar to the craving I felt for food. However, one cigarette would cure her craving, it took a large meal--like a restaurant meal plus--to make my craving go away. The light came on for her then......she said, "I guess the brain of a big person is wired differently because I never want to eat unless my stomach is growling." Of course, this lady WANTED to understand. She feels compassion for people who struggle with obesity and sees it as something she is very glad she doesn't have to deal with. So, realize that there are people out there that have a good attitude and will be supportive.
Your coworker, sounds like a mean, judgmental person who basically needs to F&$@ off. You were nice. I would have said something very rude like, "funny, I don't recall giving a crap what you think" or something equally childish. It amazes me that some people feel free to vomit their opinions about your personal life all over you like that.
Stay strong!