I think for the most part people are just too polite. I know I never comment on people's weight loss myself, unless they are specifically talking to me about it.
For me, I lost a ton of weight before people starting commenting. I'm talking probably 40-50 pounds. And then it was all negative comments. "Are you sick?", "Don't you think you've lost too much?", "When are you going to stop?", "You won't be able to maintain such a low weight." Yadda, yadda, yadda.
What I learned is that it's apparently okay for thin people to be thin. They are even envied and admired. But it's not okay for larger people to get thin. If we get thin we're "too hard on ourselves", "too strict" with our food intake, and "look unhealthy". Good thing I wasn't doing it for them, but I sure wish they would have kept their mouths shut. Funny how none of them had anything to say when I was ballooning up several sizes the year before.
The paper towel analogy is a great one. You can read the whole version of it here:
Paper Towel Theory by Bob White