That's pretty rude and mean. I can't understand why people have to be nasty to others that are doing them no harm. The best thing you can do is ignore them, treat them with the disdain they deserve. It's their bad karma after all.
I probably wouldn't have been able to resist reacting to their stupidity and then I would have been annoyed with myself for even allowing them a second of my time. So I think you did the right thing by saying nothing. You can't reason with these idiots anyway.
Don't feel bad, at least you are actively working on your weight and can look forward to a time when you're exactly what you want to be. From the sound of it, those rude people are certainly not working on their minds and there won't be any improvement there any time soon.
Take Care
I had it happen to me at my work too. I went to HR about it. HR said it is the same as making a statement about someones race or sex. 
