When your cities reach 24.000 person(s) as the final requirement for the "Palace" upgrade (limited for certain factions), there isn't reason to continue increasing the population, especially since unrest is a major issue for a city that size. Cities with low population will require more people for it to access more city upgrades.
So here are tips on how to "exterminate" your unhappy citizens to a more reasonable city size, or increase the speed growth.
Population Control - Exterminate:
Population Control: Baby Boom:
- Introduce the Plague;
Careful though, your generals, spies, diplomats and units will also be affected. There is a slight chance they die in the process (though affected units will be cured from the plague after several turns). This approach is more interesting to introduce to enemy cities than your own. But if the plague comes naturally to your city, your general will not be affected.
- Give over-populated city to enemy faction;
What for? So you can attack it and then choose the "exterminate" option. Genocide does wonders in population control.
- Placing governors will large cities means that the city will receive additional population (slaves) per turn. Removing a governor from a city will turn off this ability. Then again, you'll have to have a large army to deal with the unrest without a governor...
Depending on the patch installed, some factions will always accept a diplomatic offer of a gift. Later patches are quite harder. You will have to offer the city and some money. 3000-5000 Denarii should do the trick, depending on your relations.
- Peasant Transmigration
Create peasants from larger cities and send them to the smaller cities. Disband the peasants into the local population. Notice that unit sizes are larger if you choose that option in the main menu options screen.
- Enslave Population
The most general way in obtaining population is by enslaving a rival city. Cities with governors are the only cities that will receive slaves.
- Temples and Ancillaries
Certain temples and ancillaries will increase population growth. Pay notice to different bonuses they have to offer to the population.
- Bribe Factions and Rebels
If your loaded with cash but not with population, you can bribe rebels and incorporate those units into your small cities. This also works with other factions. Note that only units from the same race/culture may be incorporated into your cities.