Yes, we should increase health care costs for increased BMIs, because these people cost more. In the health care system, like in other systems, people should pay in proportion to the number of services they use. People who are overweight are going to use the system more often, and consequently they should put more in.
Raising the cost of health care in relation to your BMI might motivate some people to get into better shape, but if health care costs were to be raised based on BMIs then that would be a violation of basic rights. It would be discrimination against people with higher BMIs and discrimination of any kind is bad.