Nobody would dream of telling someone who got a haircut how much better their hair looks now- so good I almost didn't recognize you! This would be the epitome of rude. Weight seems to be a domain where it is socially acceptable to comment and give one's unsolicited opinion. I don't mind someone looking to be more healthy asking an honest question about what is working for me. Every other day comments on my weight, what I'm eating, saying I've lost too much or might blow away/etc. is unwelcome and intrusive.