It's not just about the relative costs of transporting overweight people. I am 180cm tall, and 70kgs in weight. On a non-stop flight between SF and London, I was sandwiched between two massively overweight men. I could not even eat a meal, and struggled to turn the pages of a book, since these two took up at least 30% of the space that should have been mine. I asked the stewardess to move me because it was so uncomfortable, but she claimed there were no free seats.

It's obvious that there need to be over-sized seats for over-sized people. I don't particularly care if they pay more or I pay less. Flying cattle-class is painful enough without being sandwiched between people who are much, much wider than the average person. It can't be comfortable for those fat people either to be crammed into a small seat. But at least they book flights knowing what they will endure (and they could book the more appropriately sized seats in business class). The rest of us don't have any determination in these events.

As for it being unenforceable, people get removed from planes for many reasons. I was recently on a plane where a man was removed from a 5 hour flight; he stank so badly that no-one could sit within 3 rows of him.