Im still pre op so i haven't experiened and dramatic weight loss but when i do and i get those types of questions i think i would make a joke out of it.
Reply with somthing real sarcastic like " what are you talking about i haven't lost any weight" Then just walk away leaving them looking stupid.
It worked for me when i quit my job to have my baby and went back to work a year later 80 pounds heavier. Everyone wanted to know how i could gain that much weight that fast. I would tell them things like oh no i haven't gained any weight my doctor said its just Water. After a while they stopped asking because i wouldn't give them the response they wanted.
As adults i think people should know what kinds if questions to ask and when its appropriate. And if they dont know i dont dignify their question with a answer.