I'm not sure exactly what problem you mean by overcrowdedness.
Are you just trying to keep your city size down for some reason? There are some micromanagement (worker / settler factory) reasons to do this, but in general larger is better except for the next thing I'm about to mention.
Or is the problem that your citizens become unhappy when your city gets larger?
To keep city size down:
- Build settlers or workers.
- Use workers to adjust the terrain around the city (change irrigation to mines).
- A city not on fresh water needs an aquaduct to grow over size 6, and all cities need a hospital to grow past size 12, so not building those structures is a way you can cap the city's size.
- Workers can be manually assigned on the city screen, and either made not to work at all or set to work terrain that produces less food (which will reduce, stop, or even reverse your city growth).
To make citizens happy:
- Acquire luxuries.
- Set the luxury rate on the F1 screen (below the science slider).
- Assign workers on the city screen to be entertainers.
- Build city improvements like temples, colosseums, and cathedrals.
- Build wonders that increase happiness throughout your empire.
- In some government types, having military units in the city will create (force...) happiness.