... what if we applied a carbon tax to fat people?
There's a weight to fuel requirement: a car with a solo driver, vs. the same car with all 5 seats and the trunk filled result in different fuel consumption. Ditto planes.
Therefore, we could charge fat people a carbon tax based on the difference between their optimal weight and their present weight, and let them eat the difference.
We could butter it and slide it through by making the carbon argument.