People just seem to think they have some sort of right to comment on others weight, or what they are eating etc etc. I have a history of eating disorders, and so I am hyper aware of my weight, what I'm eating etc. When I was underweight (not emaciated, but BMI 17/18) everyone seemed to think they had a right to tell me I looked too thin, should go and eat, and so on. I hated it, and always thought that it was strange that is is somehow more acceptable to tell someone they are too thin and should fatten up a bit than it is to tell someone they are too fat and should lose some weight. I know that the people who were saying things to me would never have been so rude as to say that to someone overweight, but if you are underweight everyone just thinks that it is their right to comment.
As you gain weight you then get countless comments about 'how well' or how much better you're looking. With these, I know people mean well, but it's incredibly hard to hear, and virtually everyone with eating issues will take it to mean how much fatter. I know that isn't what is being said, but it is what the listener hears. I just didn't want anyone to comment on my weight, and I expect it is similar for a lot of overweight people with eating issues when they are losing weight - you just don't anyone to say anything, even if it is well meaning, because you don't want to be noticed.
What really, really winds me up though, and I think is just rude, is when you are waiting to buy something (say a chocolate bar or cake or something) and someone says something along the lines of 'Oooh, you'll get fat if you eat things like that!' and then laugh. Tends to be older men who say things like that, and it is just offensive to be honest. It is not your business what I buy, eat, or weigh! And ironically I still got comments like that when everyone else was telling me I was too thin and needed to eat more 