Just got back from an eight day, 3000km snapshot of lower WA. Flight over was average but the flight back was well below. We jumped on some cheap Jetstar flights, $129 Sydney to Perth and used the trip to catch up with some friends and see a bit of the South West. The flight home was supposed to leave at 11pm but was an hour late. We paid a few dollars more for exit row seats, window and middle. My wife took the middle, she is only 60kg so the middle is usually no problem but unfortunately the isle seat was taken up by a woman that was large enough to take up two seats. She fell asleep as soon as we took off and spread into my wife's seat, she then proceeded to snore and fart for the entire flight to Sydney. We have flown a lot the past four years, visiting about 25 different countries but this was the worst flight by far.
I believe the airline has a duty of care to its passengers to make sure that another person can not infringe on the space that a passenger has paid for. If a passenger is too large to fit in a standard seat they should be required to pay for business class if available or buy two seats if it is not. If this is not suitable to them then they should take the train.