Its just a matter of contract law really, if the airline fails to provide you a service which you have paid for i.e. a seat on an aircraft not 60% of a seat due to a large passenger then what you need to do is write them a letter basically explaining how they have defaulted on the contract and what you expect in return.

I say large passenger not cos I give a toss about being PC, it just the same would apply to any person who has broad shoulders. You wouldn't call Mike Tyson a fat bastard to his face nor would you want to sit next to him on an economy seat flight.

If comfort is more important to you than money what you should do is take to the cabin crew and explain that you have a medical history of suffering shoulder dislocation make up some sh*t about being in a car crash or something cos its well known that a dislocated shoulder is a persitently weak injury i.e. the chances of it re-occuring are high...

...congratulations you just got upgraded, thanx fat bloke. Just don't take no for an answer it will cost the airline an absolute packet to miss their flight slot and have to unload your hold baggage, but do not show any sign of anger towards the cabin crew be calm and clear.

My Dad just went to London for a couple of weeks, which, considering we live in Australia, is a long way. I myself went to Brazil last year via South Africa (the long way).

What we both do, is get in the seat, chill out, then treat the cabin crew very nicely (especially the purser), and then ask if there are any other seats available to change.

So when Dad went to London he got stuck between a bunch of screaming babies, so he asked the cabin crew if he could move, and shazam, upgraded seat

i was on the way back from brazil and didnt care about the group i was travelling with anymore (and i really didnt like the guy next to me) so i told the purser the guy next to me keeps falling asleep and moving in his sleep and hitting me =P she got worried so she had a quick look around and said the only available seat is in business (and on International Flights, business is those big booths where you can lie down, spread out, have some privacy etc etc. I played it up that i didnt want to get her in trouble and she said no worries

I was stoked

but usually, if you are nice to the cabin crew, and tell them the truth that the Bus dressed like a person sitting next to you is making you very uncomfortable, they will usually move you. its only if there are no empty seats that you will need to lie, but even then its not a certainty... in that case, take a sleeping pill, and hope they need to go to the toilet while you are asleep =P

or we could just exterminate fat people...