There are a few basic factors of so much obesity now:
1. Envionmental pollution. The human body sequesters toxins it can't expel in fat to protect the organs. The more polluted areas are generally the poorer neighborhoods and communities.
2. Diet, especially sucrose and fructose, and cheap processed items that are labelled as food but contain mostly preservatives, toxins, and sugars instead of nutrition, the basic poverty diet. Feast and famine cycles of financial comfort and poverty or incarceration also affect weight control.
3. Physical inactivity of sedentary lives.
The pollution will remain for thousands of years after the industrial age. Diet may become healthier as food processing, imported sugars, and monocropping disappear. Life will be less sedentary as cars, office jobs, unemployed income, and other modern conveniences disappear.