Why do people have to make unsolicited comments on anyone's appearance, in the first place?

Unless you know that another person actively LIKES and WELCOMES compliments, don't say anything.

Regarding your example of bony men...this could be one reason why many super-skinny men and many "average"/heavyset men tend to hang out together -- both are accustomed to receiving microaggressions for not conforming to that muscular/athletic standard of the "ideal" male body.