I don't see why the fat shouldn't be taxed - have a special 'fat-tax' on KFC, McSh1te etc..., so that those who eat this type of 'food' regularly, and they are almost invariably fat, end up paying more.
As many people argue that smokers are taxed to pay for their treatment on the NHS, then why shouldn't the fat be taxed for their treatment as well.
It is well documented that smoking affects health, and therefore they are more likely to need the services of the NHS than a non-smoker, but it is also well documented that fat people are likely to need the services of the NHS for illnesses directly attributable to their obesity.
Seems fair to me.