I don't think the implication was that all Americans are fat, which is clearly absurd, but that on average Americans weigh more than people in many other countries, including the countries from which their ancestors came. And I think don't think that is actually in dispute.

Would it have been offensive if the thread had been "Why are Americans so big?" Is the trouble the word "fat"?

There is a distinction to be made between people who try to stir things up with posts that are meant to insult or antagonise others, and people who may use "provocative" headlines to draw curious people in to threads which are meant to help exchange information, or explore topics. (And both the threrads mentioned in this thread so far undoubtedly fall into the latter category, I feel.

Recently Rick Fielding has started up some great threads about various guitar chords, using eye-catching headlines that suggested they were about other stuff. For example: "What the 'F' is goin' on at Mudcat?", "Mudcat Seems To Be Diminished". I'd hate to feel that this kind of thing should be frowned on.

One of the pleasures of browsing around the threads here is saying "I wonder what the hell that is about?", and opening up to find out. Sometimes you are agreeably surprised, and you stay and read on and maybe contribute; sometimes you are disappointed, and you move on.