[mjb]

Using the model from the article, about 33% of the ticket price is total fuel cost, and 16.5% ($1300 of $7900) is 'weight marginal' fuel cost. Combining these two gives us 5.4% of ticket cost on a full airliner is weight marginal fuel cost.

Extracting 5.4% of the ticket price and making it pay-by-weight (or, rather, by the proportion of the total margin weight you make up) seems fair. On the other hand, 5.4% is a pretty small number compared to the other costs associated with "pay by weight". Lots of people would be, justifiably or not, angry about paying more because they are fatter, taller, or not a child even if that is only a few dollars per hundred on the ticket price.

On the other hand, as a tall person I would strongly support being able to pay marginally more for more legroom. Many airlines do this with a tiered model ('premium economy', 'extra comfort', etc), and it works really well for me.