I think if it's the same person all the time, be frank with them.
Tell them it bothers you. And that if they are genuinely worried, you want to reassure them. Then do so with the facts you're willing to share.
And then tell them you want them to stop. Ask them to promise that they'll remember this conversation and stop bringing up the issue. Don't drop the convo until they do.
Then, when they forget, you can say, "Remember--you promised you wouldn't bring this up anymore!" Expect them to need reminding, and don't hold it against them anymore than you'd think ill of them because, oh, biting their nails is a hard habit to break.
If it's total stranger, or people you won't see again, smile in a tired way, and say, "thank you for your concern. Excuse me--we were talking about this merchandise return?"
(I'd be asking all my coworkers, family, church members, etc., to just not talk about my weight, PERIOD. Especially after a long time of it.)
And congrats on your achievement! That's an enviable accomplishment, to work so hard and successfully toward that goal.