"It has been shown that cities of a high density, such as, for example Hong Kong, have a far lower transport energy demand per capita than low density cities such as Houston, by a factor of 18. On average, when comparing 10 major cities in the US with 12 European cities, European cities are five times as dense but the US cities consume 3.6 times as much transport energy per capita. The conclusion often drawn from such data is that dense cities are low energy cities." [1]
You are correct that there is more immediate impact to the environment from dense urban areas, but it's outweighed by the gains in energy savings from sharing resources (energy, transportation) effectively. This is why contemporary urban planning tends to push compact, dense neighbourhoods over suburban sprawl.
[1] http://www.sciencedirect.com/science/article/pii/S0378778802...
