More compact development can reduce transport emissions by an order of magnitude. Short overview.

Higher density leads to lower transport energy use per capita. The relationship between urban population density and transport energy use is inversely related (Peter Newman & Jeffrey Kenworthy).

This comparision of Copenhagen vs. Los Angeles shows this transport-urban form nexus very clearly. Both cities show similiar GDP per capita, yet dramatically different modal shares and transport energy usage per capita.

Total population size is not the reason why this is the case. Here is another example - this one by @LSECities

If you want to get a better grasp at urban mobility and sustainability issues I can recommend this @BartlettUCL and @TUMInitiative e learning course starting next week
https://www.futurelearn.com/courses/components-of-sustainable-urban-mobility