Not sure why you're gone all grey there. This is the prevailing thinking in obesity research at the moment. You're more likely to be obese if you're poor than if you're rich.
You don't get as much chance to die from obesity related diseases if you've died in a war, famine, pandemic, or from coal lung or crushed to death in a factory accident.
It afflicts the poor in relatively comfortable and safe nations.
In other words, obesity comes from worse diets. The working classes are bombarded with corporate foods that contain way too much sugar, etc.