We can go back and forth about whether or not you are a "happy" overweight person or not. From my perspective - it is difficult at best to carry around significant excess weight daily and be active, notwithstanding the fact that there are various associations of heavy people who claim they are fit. But if and when you are placed in a situation, for example fitting in a traditional sized movie seat or plane seat - at what point does the other person's comfort become your issue?
The New York Post printed a photo in its December 3rd edition that showed an extremely large person, it appears to be a man, sitting in an aisle seat. He is so large that half of his body spills into the aisle, almost touching the seat across from him. This appears to be a plane with 3 in a row, only 2 sections on either side of the plane (a 757). The source of the photo is a stewardess who took the shot from the back of the plane, while still on the ground. American Airlines claims it cannot at this time verify authenticity of the photo - meaning that there is a chance it is photo shopped. Let's assume it is the real deal.
There are a couple of airlines that currently release authority to the check in service desk to "assess" flyers for their size or girth. In the case of a suspected issue, the attendant can do a seatbelt size check and if the person cannot possibly fit the seatbelt across their body, they are offered (a) an extender or (b) the need to purchase a second seat. In some cases an extender doesn't work so the "purchase a second seat" or else not fly is the only possibility. Of course, many airlines feel uncomfortable dealing with the issue and there is no doubt that even when the airline has established the evaluation policy at the check in desk, the airline attendant may not want to raise the issue - after all, you are in a sense, perceived to be making a judgement. Once on the plane, the seatmate may feel very uncomfortable complaining or making a scene.
On the other hand, someone who, in good faith, purchases a seat, has the expectation of having some portion of the arm rest as well as the full comfort of their seat. If an extremely large person sits next to them, chances are, the space is encroached upon and some level of expected comfort is lost. No matter how long or short the flight - someone who paid for a certain amount of space, has not gotten their value and may have felt really disadvantaged.
So, should we expect a person who knows that they need "more space" to be proactive and avoid inconveniencing others and the possibility that they may have to deal with a somewhat humiliating experience at the airport? Should we put the burden on the airline? On the desk or flight attendant? In the case of this photo - the safety of many passengers may have been put at risk, since it's clear this passenger's girth extended dramatically into the aisle space. Attendants would have also had a problem passing drinks, answering seat calls. And passengers would have had difficulty getting to the restroom and getting off a plane in an emergency.