If a city’s population doubles but incomes stay constant, the city’s floor space per person declines by 40%; If per capita income doubles but population stays constant, the city’s total floor space per person increases by 29% - these are just two key takeaways from the new World Bank report "Pancakes to Pyramids : City Form to Promote Sustainable Growth" that draws on new evidence, econometric analysis, and predictive modeling to relate the economic growth of cities to their past spatial evolution. Which one is more sustainable for urban future - horizontal growth or vertical layering?