I once incurred the wrath of many on a forum for simply saying that my best friend is quite fat.

100% of the those that were angry were from the U.S.A. and this was pointed out in the discussion by others that didn't seem to see the problem with stating such a simple thing about my best friend and most of those that were angry relented after chalking it up to a cultural difference.

But apparently it is deeply sensitive in the U.S.A. to remark upon a man's weight being too much.

The last time it happened to me was someone's remarking that I was putting on some weight, and I agreed, so I switched to a diet with more fibres and in mere weeks it was fixed. — it seems quite constructive to me.

Yeah I lived in cuba for a work project. I travelled to another city for a month, and when I came back the head of my host family said “you’ve gained weight”. Was probably five pounds or so.

Was motivational to stop that weight gain in its tracks! At that level pretty simple diet changes will turn it around.

In American culture being fat is seen as a moral failing. Calling people out who already know they are overweight is offensive. You aren’t telling them something they don’t know. Likely they have already tried to lose weight and failed.

What's wrong with calling people out for a moral failing? It's more polite to ignore and pretend it hasn't happened, but it's a bit insane too, isn't it?