The current trend in urban planning is to "build the city on the city" ie to increase the density of population rather than to expand the surface used by cities.
Expanding the surface used by cities increases car usage, thus pollution and oil dependency, areas used by roads, buildings, etc... thus water streaming and gullying. Having low density population areas increses the need for individual cars and reduces the profitability of public transport systems.
On the contrary, increasing cities density allows more people to rely on public transport thus reduces individual car use. It allows for a more efficient electricity, water and sewage networks. It reduces the area of polluted soil.
All these aspects could be developed.....but you see the point.