Any observer of commercial aircraft over the past few decades cannot fail to have noticed that the size of airliners has dramatically increased, particularly for the longer ranges, hence the A380, Fig. 13.12. The reason for this is quite simple. Apart from the wings, contributions to the overall drag of the aircraft come from a variety of other sources, including the fuselage. For similar fuselages the capacity increases as the cube of the diameter while the surface area only increases as the square. Since the drag is dependent on the surface area this means a reduction in the drag contribution per passenger, and a consequent improvement in the economy of operation.
Of course other considerations work to restrict the size. In order to provide an attractive and useful service, the airline must operate a reasonably frequent service over a given route. If this results in the aircraft having to operate with a substantial number of seats empty this clearly undoes any improvement due to the increase in size.
Other factors include the provision of passenger handling facilities at airports which are able to deal with a very large number of passengers joining or leaving a large aircraft. Any reader who has had the unfortunate experience of being a passenger on a diverted jumbo jet will have first-hand experience of the chaos which can result at the unsuspecting passenger terminal of the receiving airport.