I'm absolutely not shuffling around the question rather than answering it.

In a practical sense, the question doesn't matter at all to people trying to live healthier lives.

The reason we have fatter people is we have more calorically dense foods readily available (meaning greater calories in) while people live more sedentary lifestyles (means less calories out).

This function to lose weight is so incredibly simple (paging jacques chester and his steam machine blog post).