Nearly all airports started out small, regardless of their original purpose, military or not. I use LHR, FRA, and CDG most of the time. Each uses busses to transport passengers to distant, and not-so distant airplanes, followed by a 1930’s style climb up the stairs to the plane. LHR’s design, completed in 1946, had no parking space incorporated – ideal for the time, not so much now as parking takes immense space. FRA and CDG are huge, as FRA was a huge startup military airport. They are still expanding, and still can’t get the bus out of their designs. CDG was developed, exclusively, as a civil airport and is huge, covering more than 12 square miles. All modern and all fairly recent. Busses everywhere. For space constraints you just cannot find more constraint than at EWR or DCA, for example. No busses, no climbing stairs, at either. The situation may begin with space constraints in some cases, but it goes beyond that.