The last time I complimented someone on their weight loss, she told me she'd been very, very sick and was still losing weight and couldn't stop.
So unless I know someone is deliberately trying to lose weight, I keep my mouth shut.
There is a lady who works in the pharmacy department at my local grocery store & she is the sweetest thing on two feet. She was so very, very helpful when my dad was in his last days, going out of her way to get special meds that he needed. About a year ago I noticed she was losing a lot of weight, and I was so wanting to compliment her but was afraid to say anything lest her situation be similar to the one mentioned above. So I didn't, even though she looked great. I'd guesstimate she lost about 75 lbs, and she lost it QUICKLY.
She's now gained most, if not all, of it back. Again, I have no idea if this was deliberate on her part or not. But if it was, it is so sad to see her lose and then regain so much weight in such a short period of time. She is definitely overweight again - probably in the obese category.
I now wish I'd said something encouraging to her. But again, I was so gun shy because of my previous experience.
Also, the last time I dieted (in 2010) I lost 30 lbs. I'm 5'2" tall and it's what I needed to lose - brought me from overweight to normal. It was a big difference in my appearance and not a single soul ever said a word to me about it.
So there you go.