Too Big For An Airline Seat

My most recent airline flight was uneventful except that I noticed a growing problem on planes, people who are too fat to sit in their seat without encroaching on nearby passengers.
In an aisle seat, two rows to the left of me was a man who occupied the aisle seat of the two seat row, the seat adjacent to him not yet filled as we passengers boarded the plane. After the boarding had been almost complete, and the extra seat remained unfilled until a young woman approached and stopped, relating to the 170 plus kilo passenger (who spilled into the unoccupied seat due to his beefy physique) that the empty seat was hers. He quickly pointed to an unoccupied seat in the next row and asked her if she wanted that. She did. Problem averted.

But what happens when there is not alternative and the passenger who purchases a ticket must ride with a heavy passenger slobbering into his or zone? Should the airlines make a too big passenger buy the adjacent seat? And if so, what standard determines who is too fat to fit in one seat?

We all are seemingly getting fatter these days as the airlines makes seats smaller to make more money, and the growing number of people who are too fat to fit into tiny airplane seats cause discomfort to their seat mates. If a passenger pays for a full seat but has a seat mate spilling into a third of it, is that fair? Among American airlines only Southwest Airlines currently has a rule about too fat riders having to buy the adjacent seat (and few grossly obese fly that airline as a result), and there is no FAA regulation about the problem. Too, the more I fly the more I see this problem occurring.

Conversely, is being too fat a handicap that should be treated as a legal disability? If so, this would make mandates for airlines to set aside a section of extra wide seats to accommodate the extra large passengers (who are, ironically, probably far more uncomfortable than the passengers they spill on top of). Maybe the jumbo passengers who fill the bigger seats could pay a little extra for the bigger seats so slimmer passengers would not feel slighted.

At the very least, the FAA should require all airlines to publish clear policies stating how they will protect squished and uncomfortable passengers who find themselves getting less than what they paid for. What do you think?