LONDON HEATHROW is one of the world’s busiest airports. It is also remarkably small. Pascall+Watson, a firm of architects, compared the ground area of the world’s 15 busiest airports and plotted onto their outlines new and planned additional runways and terminals. Denver, which sprawls over 135.4 square kilometres (52.3 square miles), was the biggest of the 15 but also the least busy, with 52.5m passengers in 2013. Heathrow, in third place in 2013 with 72.4m passengers—after Atlanta (94.4m) and Beijing Capital (83.7m)—was the smallest, occupying a site of just 12.3 sq km. Nearly all the other airports also have more runways, often many more.

According to Alan Lamond, Pascall+Watson’s aviation director, this shows the effects of decades of indecision over expanding Heathrow or building a new London airport. Heathrow has become extremely adept at squeezing every bit of capacity out of its restricted size, congested terminals and its miserly pair of runways. But efficiency only goes so far. In Europe, Paris Charles de Gaulle, Amsterdam and Frankfurt have more space and more runways with which to attract additional services and passengers, including those interchanging between flights. Big new global hubs are emerging, such as Beijing Capital and, especially, Dubai—which leapfrogged Heathrow to become the world's busiest international airport in 2014. In 2018 a new international airport is due to open in Daxing, a district south of Beijing. Costing more than $13 billion and boasting four runways, it is designed to handle 72m passengers a year. A new five-runway, $32 billion Dubai airport will have a potential capacity of more than 125m passengers a year, in addition to the existing international airport.

Plans to build a new London airport in the Thames estuary have been proposed on and off since the 1970s but now seem to have been permanently shelved. Even if controversial plans to add a third runway at Heathrow ever get off the ground, the airport may never be able to grow big enough to compete on its own terms with these new hubs. That may mean Heathrow has to work as an airport system with the other London airports—Gatwick, which played host to 35.4m passengers in 2013, and Stansted, with 17.8m.