People dont gain massive amounts of weight between the time they book their ticket and the time it comes to fly... (unless its a 6 month in advance booking!
). Whilst I have utmost sympathy and compassion for those of larger stature and their needs, its not fair on pax around them to have to deal with something that is not their fault (although I do acknowledge that wieght problems can be medically induced).
Southwest has a good policy regarding "Customers of Size" here
http://www.southwest.com/travel_center/cos_qa.html. This bit is a interesting definition:
What is the definitive gauge for a Customer of size?
The armrest is the definitive gauge, as it serves as the boundary between seats. Customers who are unable to lower the armrests (the definitive boundary between seats) and/or who compromise any portion of adjacent seating should proactively book the number of seats needed during initial reservations.
From what I can tell pax are required to purchase an extra seat, and if the flight does not sell out they are entitled to a refund of that seat. I think thats a very fair policy and one I would like to see implemented at more airlines. I believe they only get charged a child fare for the 2nd seat?
There are times when you get "customers of size" who feel they have every right to demand an extra seat as they take up more room... sorry folks but you get what you pay for (and that includes those next to you - they pay for a seat so they are entitled to it).
By the way I am not lacking compassion or understanding here, but its something that needs to be proactively dealt with by both pax and airlines.