<i>sarcasm</i> And clothes for size small should be cheaper than size large, since it has less material.<p>To make this work in practice is really hard. Do you charge people at the gate? If a teen bought a round-trip ticket to be a high school foreign exchange student, and had a growth spurt while training for the football team, then does the teen need to pay more money in order to return home? And if that student doesn't have the money on-hand at the airport? At least with luggage you can leave something behind; with your body it's harder.<p>I suppose here the student who gained 10kg could leave 10kg of luggage behind. That's about 1/6th of the total weight allotment for check-in + carry-on. But suppose the weight gain was more than the luggage carried. Do you want to be the airline which refuses to let a student return home?<p>Suppose someone prepaid the expected weight then lost 10 kg. Do they get the money back in cash? After all, the person might not have a credit card, and depositing a check in another country can be really expensive. In the case given (US to Australia) would that rebate be given in US or AUS dollars? Is it taxable? If you travel for work then does the company get the money back?