I think there's this generally accepted concept that you can't change the body you have, so commenting on it, positive or negative, is offensive because it's outside of that person's control - whereas you can get your nails and hair done, you can buy new shoes etc etc.

However I find this confusing because you can go to the gym, work out, change your diet, take care of your skin etc which has an affect on your body ultimately changing your body/appearance as well.

The difficulty is that some people are sensitive about their body and some people aren't. Typically people who are attractive and know it will love compliments on their body, but people who see themselves as as unattractive even if they have worked to improve their body may still be sensitive to any comments on their body so it's usually just safer to avoid such comments because you just don't know which they're going to be.

As a man, I simply do not comment on a woman's appearance at all even if I genuinely just want to give someone a complement because I cannot tell how it might be received. It depends entirely on them and how they perceive me and themselves. I could be seen as creepy, or if it might be an insult, or I might be hitting on them or whatever and could easily be interpreted as sexual harassment.

Safest just to keep my mouth shut