I've probably lost a little muscle tone in, erm, 'the chest area', and I suspect that that's what they have noticed. Even so, I still think it's rather impolite to publicly note a change in weight either way, whether or not someone's been ill. I mean, if someone had cancer, "You've lost weight" would be more than a little insensitive. If someone's been ill, saying they look ill would be extremely crass, and even when someone looks well, saying "You look well" can be really upsetting for the person if they really aren't well.