Peter Singer, sensitive as ever, considers the idea while at an airport:
A slight Asian woman has checked in with, I would guess, about 40 kilograms (88 pounds) of suitcases and boxes. She pays extra for exceeding the weight allowance. A man who must weigh at least 40 kilos more than she does, but whose baggage is under the limit, pays nothing. Yet, in terms of the airplane’s fuel consumption, it is all the same whether the extra weight is baggage or body fat. … [T]he point of a surcharge for extra weight is not to punish a sin, whether it is levied on baggage or on bodies. It is a way of recouping from you the true cost of flying you to your destination, rather than imposing it on your fellow passengers. Flying is different from, say, health care. It is not a human right.
James Joyner dismisses the idea:
If we take Singer’s data as face value, added passenger weight has a negligible impact on fuel costs. $472 in fuel costs on a large airliner (which seats 525 people) on an incredibly long flight is next to nothing. After all, a flight from London to Sydney is billed at more than $2000 per passenger. And, frankly, it would almost surely cost more than $472 per flight to carefully weigh, assess, and haggle with 525 passengers.