I lost about a quarter of my body weight a couple years ago (and gained it all back in the last six months
), and I got a LOT of comments about it. Friends knew I was craving the positive reinforcement, coworkers asked my coworker BFF if it was okay to say things to me, and random friends of friends usually just blurted it out. Most of the time I was thrilled with the comments and was happy to talk about it, but it's all about the phrasing. "You look great, have you lost weight?" is so so different than "OH MY LAWD, you have lost SO much weight, you look AMAZING!"
In this scenario I probably would have blinked a few times and simply said, "Oh, I didn't know I was supposed to say anything. Good job, you."