It's their personal responsibility, unless they can prove they have a health condition like hypothyroidism, they should be taxed more.
Obesity leads to hypertension, CVA, MI, diabetes, arthritis, OSA, erectile dysfunction, high cholesterol, etc. then they goto the emergency room for their MI and costs taxpayer money. These are avoidable health issues and a huge burden for the entire nation.
You would obviously titrate it...you weigh this much...unless you weigh this much at this point in time you will be taxed more, then you meet another benchmark. You can use weight, you can use bmi, etc.
Hurt their pockets, they'll eat less and consider their health and the fact their kids might not have a father because their poor eating habits which they then pass down to their kids.