>1. Historically being fat was considered a sign of good health. Of course, this was during times of massive food scarcity and the definition of fat was probably very different from what we consider fat today.

I'm surprised you put that in and not:

4. motivated reasoning. People want reasons to justify their current lifestyle and not having to put in hard work to change it.

For point #1, most western cultures already idealize thinness. Therefore that's not really a factor unless you're from a culture that idealizes being fat.