Folks that are taller often make comments like that, as Quiet Ballerina suggested. They think of how that weight would look on them as reference. Others may be jealous. Sadly saying someone looks anorexic does seem to be what some throw out as an insult. Some genuinely may be worried about your weight according to what they're use to, like parents, especially with anorexia/bulimia being so common in today's world.
In any case, definitely focus on getting healthy, and make sure they're aware that's what you're doing, and the medical side of things supports that weight. If you think they're genuinely concerned, maybe get together some of the medical paperwork that shows that's a healthy weight for your height, just so they're educated as well. At least that's what I'd try to do. As for the people just trying to be insulting.. I don't have a witty comeback. Ignore 'em, they're jealous