That 6% number isn't even close to accurate. There are many other very expensive chronic conditions that are downstream of obesity including type-2 diabetes, heart failure, hypertension, MSK injuries, etc. We are digging our graves with our teeth.
Except all those things reduce peoples lifespan, and age-related care is by far the most expensive medical care.
Although I tend to think we overwork the working class such that they have no energy to keep up their health, so this would basically be taxing them because they're poor in many cases.