Measuring the world’s largest airlines can be done in several different ways. For example, one could measure an airline’s size by statistics like available seat miles (ASMs), revenue seat miles (RSMs), fleet size, or number of destinations served. If one were to measure the world’s biggest airlines by these metrics, United Airlines would almost always come out on top. There are multiple other ways to measure the prominence of an airline, which will often result in other airlines being the world’s largest.