In recent years there has been a slimming down, as it were, regarding freighters – a turning away from the jumbo 747s and 777s and toward the squat 737s and svelte 757s. The e-commerce revolution, especially in Asia, requires smaller freighters to make more frequent stops. But because there are no current programs to build narrowbody freighters from the factory, customers must rely on companies providing passenger-to-freighter conversions.