Maybe in a lot of US/European airports that originally date from the first half of the 1900s, they're too close to the city center and don't have enough land to

Maybe in a lot of US/European airports that originally date from the first half of the 1900s, they're too close to the city center and don't have enough land to expand. Even then, they seem to be regularly able to find ways to get more gates in renovations (ex: Newark/EWR's Terminal A replacement is going to have the ability to be expanded for 12 more gates). I'm sure there's an upper limit, but plenty of those older airports can find space for more gates in modern redesigns. Most of the big modern airports were built with massive expansion in mind. Well outside the city centers and with large land areas that they already own (and so won't be developed).