I think I agree with the sentiment, but being overweight is a characteristic of someone’s body. A fat arse is a part of a body. You can be overweight without a fat arse and vice versa, those are not identical.

I would naïvely think that “fat arse” is vulgar or insulting enough (or at the very least way too informal) not to use it with someone you’re not familiar with. This has nothing to do with political correctness.

You can ruin some people’s day by calling them ginger. I would expect a psychologist to be sensitive to this sort of things (do no harm, indeed), but that’s not a good reason to make everyone stop using that word.