Fuel costs are essentially linear with regards to weight, and accordingly dominated by putting several hundred tons of metal in the air. After you've made that

Fuel costs are essentially linear with regards to weight, and accordingly dominated by putting several hundred tons of metal in the air. After you've made that decision, adding a marginal passenger is much, much, MUCH cheaper than the price of a ticket. The margins approach that of the last customer added to a SaaS company. (!) Airlines occasionally obscure this with regards to their public pricing rationales for extra luggage, but that is almost solely an ops/revenue-maximization problem for the airline rather than being sincerely a proportional response to the additional marginal cost of putting another bag in the air.