It has been common for airport traffic forecasts to assume a continuing increase in average aircraft size in the future so that the anticipated growth in passenger demand will not result in a proportional increase in aircraft operations. However, to understand the likely future trends in average aircraft size, as well as strategies that an airport might pursue to encourage airlines to use larger equipment, it is necessary to understand the competitive dynamics of the various markets served by the airport and the factors that determine the size of equipment that airlines will use in those markets. Recent trends in average aircraft size at a major U.S. West Coast airport and the influence of market characteristics on the size of aircraft used in different markets are examined. The changes in the past decade in selected short- and medium-haul markets are analyzed, and the results are presented of a cross-sectional model that estimates the influence of various market characteristics, including air traffic delay levels, on the average size of aircraft deployed in a wide variety of markets throughout the United States. The prospects for airport capacity constraints to influence airlines to make use of larger aircraft are discussed, and the implications for airport strategies to encourage the use of larger aircraft are examined.