Airline size looks simple until the metric changes. A carrier can board more domestic passengers while another generates more revenue from long-haul international routes or premium cabins. Capacity measures can also shift the ranking because one wide-body flight carries more seat miles than a short domestic sector.
For a stable comparison, ACR starts with Bureau of Transportation Statistics traffic data and uses company filings for revenue and network context.
The big four have different network models
American, Delta and United operate large hub-and-spoke networks with significant international businesses. Southwest remains more concentrated on domestic point-to-point and connecting traffic, with a fleet and operating model historically centred on a narrower aircraft family.
Those differences affect fares, costs, loyalty economics and exposure to international demand.
Passengers are not the same as profitability
High passenger volumes can coexist with weak margins if fares are low, fuel costs rise or aircraft utilisation falls. Airlines therefore watch unit revenue, unit cost, load factor and capacity alongside total customers.
For investors and airports, the passenger ranking is useful for scale but incomplete as a measure of economic quality.
Government traffic data make annual updates straightforward
DOT and BTS releases allow the ranking to be refreshed using a common reporting framework rather than airline marketing claims.
That makes the page useful as a permanent reference point for future coverage of capacity, fares, aircraft orders and airport investment.