Surely if you don't fit into one seat you must buy two? Or is that just too obvious to be mentioned?
If that's deemed not fair I'm going to bring a 40Kg suitcase along and demand its allowed on foc because I can't help it if my baggage has a glandular problem and it has rights...
As an afterthought perhaps there is an opportunity here for the pikey? Charging pax by the Kilo. That should get a coupla months more free advertising.