What I
would do is expect a reasonable charge if and only if the obesity required extra accomodations. Need a plus-sized MRI machine? Those are pricey, and that cost should be reflected in the bill. Take up 2 seats on the bus? You should pay for the extra seat. And so forth...
It's not even that I would implement this to "punish" or "discourage" obesity. It's simply that most services operate by offering a certain range of options, centered around the average consumer. Requiring special treatment costs extra money. Tall people have to pay for extra legroom, wide people ought to pay for extra butt-room.
If obesity becomes average, well, then companies may well adjust their standard options to suit that...but I don't see why we should start with the assumption that these accomodations should only cost the companies providing them.
People should be allowed to make decisions, even stupid ones.