I think there's a difference between someone who is large and someone who weighs over 20 stone. If you're large you can probably still keep to one seat, if you're 20 stone plus then you will clearly need more space. I see no reason why you should not have to pay for that, I don't want other peoples wobbly bits occupying half my chair, (and I'm not small myself, I'm a size 16).
However, I do think the better idea is that airlines should provide bigger seats so that if you are particularly big then you can book to sit on those. Lets face it, if you're of that size, then you know full well you can't sit in a normal seat comfortably (or probably without embarassment) so having that option would probably be more useful.