I had taken this as a bit of a laugh, but if you are serious, it is already happening to a degree.  You can carry up to 2 children on Qantas, under the age of 2 & not occupying a seat (probably between 4Ks & 15 Ks each ) for 10% of the adult fare, or an average of $130 each to LA.  If over 2, they would pay 75% of the average fare or around $975 each.  Just look at the excess baggage cost to LA, of $105 per item of up to 23 Ks.  The fuel surcharge to LA is $500, so the actual cost in fuel alone of excess baggage is around $4.50  per Kilo.  If they have a medical problem, why doesn't Medicare pay for them.  We are already subsidising them, why should normal travellers pay twice?

A bum measurer along the lines of the frame used to measure cabin carry-on luggage is the cheapest.  Just bend over and stick the tush in it, or at least try to.  Unders are normal fare, others are fat fare.

Either that or a weighbridge that all pass over with luggage and get a ticket.  With a large screen above in imperial and metric and a voice annoiuncement for the sight impaired (shopping bag over head).

Heh, heh, can't see why anyone would object to that.

How to choose the standard Aussie @rse would be the problem.  Should a celebrity caboose like Julia Gillard's set the standard?  Yay, a measure of 2.5XNSW Government railways pickhandles across, plenty of room for most.  Which only leaves the problem that they might take her squatness to set the distance between seats.  That would be cruel.  Off to the 'economist's calculator' (works backwards) to figure it out....

I would never be judgemental re anyone with looks or weight or colour or creed.

but I would be quite happy with my 8stone 10lbs to pay a bit more than my 7 stone counterpart when travelling on a plane and really wonder why anyone would have a problem with weighing (in private) and allowing the plane to know how much weight it is carrying - maybe it is me but if I required two seats to travel so be it and I would buy the two seats or pay more if I was 12 stone or more to pay the extra. My main concern is the fact it needs to be gauged how much weight is in the plane as after all hand luggage is such a big deal. I couldn't care less how much a person weighs as we are all different but just for flying I feel it may need to be addressed. Certainly not for looks or even for the person's health just for the fact the plane needs to stay in the air. er for my sake lol.

There should be warnings on all tickets and on departure doors that visiting some countries could result in weight gain and possible refusal of seat on return, or payment of a penalty.

That wouldn't apply to First Class and Julia's caboose, only to to the status seekers in business and definitely to cattle class (good God, why can't they control themselves).