My mother moved in with us a few years ago, and it has been a MAJOR adjustment - before that, she lived 2500 miles away. Anyway, she gets in my business quite a bit, and I used to let it get to me, but now I just turn it around and fuss over her: "Mom, it sounds like you are really worried. I wish you wouldn't let this bother you. The doctor says I am fine, honest." Conerned look on my face like she is a sweet little old lady (which she is) who worries too much. Works like a charm - with my Mom, anyway. She gets listened to and I get to be in control of my own life - everybody's happy.
As far as people who have no business weighing in (sorry) about your weight, I would just smile and change the subject. Even if they mean well, they are not entitled to an explanation.