Don't get into conversations with people about weight then?
It rarely comes up naturally. If someone marches over to you and says "Oi, fatty, drop the Mars bar!" then obviously that's very rude.
But usually when I get people complaining someone's 'talked about weight' with them, the one who brought it up was them. "Oh, I'm so fat, oo, I'm going to have a naughty treat, oo, I'm doing Slimming World what are you doing? Oo my food is a sin" and they go ON AND ON
and it does my head in a bit. I am sporty, I enter competitions in my sport, and I keep my fitness and nutrition stuff private, but then they come at you with "oo I bet you never eat naughty treats" or "oo I bet you've always been thin, tell me what you eat" and "I bet you're really super healthy eh" and I still don't talk about it because I really hate goading.
So. Honestly? If someone's come up to and from absolutely NOWHERE started banging on about weight, they are crazy rude and yes, you have my sympathies.
If you tend to be the one to bring up weighty matters? Less so. People who talk about it a lot, there's no harm in thinking they're digging for advice.