I've been waiting years for fatties to have to pay extra to fly. IMO they should have a weight-graded pricing system, where heavier people pay more to compensate for the extra fuel needed to haul their fat asses skywards.
My other (favourite) option is you get a set weight limit for everything; you + luggage. Hell, I could fly with my bike for the weight of the average UK person alone.
While I realise that we don't want to encourage crash-dieting caused by passengers attempting to make cutoffs, it could definitely be beneficial all round.
Have I told you about my belly tax where larger people are made to pay for the extra CO2, medical costs and scenery ruining visuals they impose on the rest of us? ;)
Disclosure: I'm 59kg despite eating like a horse. Always have been.