So in your understanding of the movie, [genetically] fat people ruined the Earth and fled in a spaceship. Interesting.

I would honestly like to know what you think happened to the thin people in this apocalyptic scenario. Maybe they were killed off by an earlier famine, or cold snap?

Maybe you should be thanking Pixar for making a movie where fat people are depicted as more genetically fit, as they are the only survivors of this disaster.