Do obese people generate costs for other people? Mainly not, according to a review of the literature:
In employer-provided health insurance pools, being obese causes limited externality harm because obese individuals likely pay the costs of their body weight through reduced wages. In public health insurance, there is an implicit transfer from thin people to obese people, but this transfer is progressive and seems unlikely to induce substantial social loss.
[One] way in which the obese “subsidize” the thin is, presumably, by dying earlier and not claiming as much in Social Security benefits…for 50-year olds, obesity reduces life expectancy by 1.65 years…for 65 year-olds, obesity reduces life expectancy by 1.05 years.
See Robin Hanson as well.