The majority of obese people have the power to change it themelves. We are talking about some excess weight that you can take off if you take the time and work at it. If they improve their diet, they will lose weight.
An obese person is already a safety issue for the rest of the passengers on a plane. In the event of an emergency, you now have to deal with some extremely overweight person slowing everyone else down if they have to exit the plane quickly.
I don't think it is a disability because they choose to be obese.