I can't help thinking that the reason we are not allowed to state true things is a reflection of the esteem of the receiver rather than anything else.
I know I've got massive bags under my eyes and people constantly say I look tired. I do! I bloody know it! And I don't consider it 'rude', it's just something I am aware draws attention.
I also have a disgusting toenail, a birth mark, a bad hair dye job, a large forehead and massive calves.
It's just stuff that people notice and I have to learn how to deal with it, I just wonder why being overweight isn't in the same category. We all have stuff about us that people notice, we either have to change these things or deal with the responses.
It is factual thst size 20 is huge. It's not shaming, not judgey, just the reality. I don't know how you can expect people not to notice it?
When people say it's nasty and rude, it is only rude if you take it that way, (presumably you also know you are huge) and so the offence only comes because you are not actually comfortable in your own skin, and therefore that has to be your problem not the person who notices it!
If I steal something I can't get hurt about someone pointing out I'm a thief. Prob bad example, but same principle
A better example might be don't bother tidying my house, I can't get offended if someone calls me a sloven