"You're so tiny, you need to eat!"
"Wow, you're tall/short!"
"You've lost weight!"
For some reason, people have begun to assume that making these kinds of comments to others is perfectly acceptable. Whether it be a face-to-face statement or a comment on Facebook, some individuals have no filter - and while I appreciate and advocate for speaking your mind, there are just certain areas where we would prefer that you keep your mouth shut: size is just one of them.
Just like finances and other personal topics, size ought to be held to the same standard as these topics when it comes to discussion: it needs to not even be a topic of discussion, as it really is no one's business. One would never openly ask what one's salary is, so why is it appropriate to ask someone what their weight is? Furthermore, it would be incredibly inappropriate to comment on someone's job choice on the grounds that it doesn't pay well, but why do some deem it appropriate to comment on someone's perceived weight?
Why should my size NOT be a disputable issue among others? Simply put: it is MY size, not theirs. The size and shape of my body and the size and shape of your body does not affect or influence the lives of anyone else but your own self, so why should it be something that is poked at by others?
Additionally, though you may not intend for your comment to be taken in a negative way, most of those receiving the comments take it as an insult because you have made it clear that their size, in that moment, is something that you were focused on. By nature, we are insecure about our own bodies to begin with, and someone on the outside drawing more attention to the body just makes things worse. Whether it be a comment praising someone for "looking like they've lost weight" or a seemingly joking one encouraging them to gain weight, the burden that both types of statements carry is the same: an unnecessary focus on physical stature.
This goes for the bodies of all individuals: someone's size is not your business. The only body that you have control or say over is your own, so stop making ignorant comments, even if you have no intention of hurting the receiver of the message: mind your own size, and I'll mind mine.