Nero did it, to Rome, to make room for his phat pad.

But who wants to be Nero. I want to be like Augustus.

Another, slightly more realistic option, settlers. Back in the day, when cities became overcrowded, many city-states would send out settlers to found a new city. This increased total revenue (because now you had two city's worth of farm revenue), solidified your people's hold on a chunk of land, and relieved squallor problems. In RTW you can't found new cities (at least not to my knowledge) but you can "redistribute" your population.

Step 1 - Jack up the tax rate in the over-populated city to bring down growth rate
Step 2 - Click on the over-populated city
Step 3 - Alt-Right click on the under-populated city
Step 4 - Cue up a bunch of Peasants (this will often increase happiness, allowing an even higher tax rate)
Step 5 - Disband the Peasants in the under-populated city when they arrive.