I'm very small and on the low end of the normal range for my age and height. That comes from a combination of genetics, a healthy diet, and a lot of physical activity. Yet people, especially the overweight, feel that thin people are fair game. For example, when I first came to Germany, an overweight woman told me, "You're so thin. You must starve yourself." My response to her was, "Come and see me after I've run 10 miles in the woods and then tell me that." I would never tell an overweight person, "You're so fat. You must sit in front of the TV all day eating Doritos," or "You must have really slow metabolism."
I've also had people tell me that they used to be thin either: a) before having children, or b) when they were younger. How the heck am I supposed to respond to that one? My natural reaction would be, "So what happened?" But I don't say it because it's rude. I have had a child and was able to get back to my pre-pregnancy weight through diet and exercise. It took some work to lose the last 10 pounds (4.5 kg) of baby weight, but I did it.
If I could pick how I look, I'd stay just how I am. I love being active and able to eat pretty much anything that I want without worrying about my weight. How many other women can say that?