It's up to the FA to avoid embarrassing the person. I certainly wouldn't go out of my way to make the person feel bad. However, really large people are very aware that they live in a world designed for people of less girth. Even if they are a first time flier, they've at least seen the cabin of an aircraft on television or in the movies. They would have to know that they might not fit into the seat and, if they don't, they'll be spreading into someone else's.

Exactly. I have nothing against people who are obese -- I judge people by their character, not by the pound. Someone who tries to take over my seat space, whether because they are obese, or are holding a lap child or a lap top, is imposing on me, and I won't accept the imposition.