Obese is the term I prefer to use, and there is a definition of that according to a person's body mass index. Most people should be aware of that definition.
Well, if we want to compare a smoker to a fat person - let's just say that a smoker pollutes the indoor air for everyone - including those who choose not to smoke. A fat person does not affect the health of others around them. Both smokers and obese people will cost taxpayers a lot of money in the long run because of cancer and diabetes etc., so there should definitely be health campaigns to try to change people's smoking and eating behaviours.
As for airline seats and other ways of accommodating obese people - yes, a certain level of discrimination is okay. If they have to make huge economy class airline seats it means less people can fly on a plane, then it costs everyone more money to fly. So, I say have reasonable sized seats and leave that as an incentive for obese people to try to lose the weight. The other alternative is to ask obese people to pay the extra money and upgrade to business or first class where the seats are large enough. I'm tall and there are only so many seats in economy class with adequate leg room (and I hardly ever manage to get one), so I am already being discriminated against for my height. Why not obese people too?