Many well-functioning huge metropoles are poly-central. A notable example is Berlin, which was merged from many cities in the 1920s, and was split by a national border 1948-1991.

Each little town acts approximately as a little city with retail, offices and services at it's center, and residential around it. Many of the 'centers' have rapid transit a connection to the two or three main employment centers, and many big cities have peripheral lines as well.

This means most of the services you'd use in your life are in your little town, hopefully reachable in walking distance, except your job. It may be located near another center, or in one of the CBDs (central business district). You'll have a walking or short bus connection to your local rail station, which in turn connects to the other centers, so most people will have reasonable connections to get to work.

One way to get to this paradigm is to leverage rail lines and subways to encourage this sort of development, building towns around stations. Unfortunately the tendency is to build transit along highways, which is cheap, but ultimately toxic to the idea of building walkable/human-scale towns surrounding the stations (see e.g. REM light rail proposal in Montreal). Another tendency is to build large parking lots around stations instead of towns.

To create and maintain smaller cities you would need to enforce controls on population growth and distribution. Controls which are abhorred by the civilized world.