Squalor and Overcrowding
In Rome total war as settlements grow so does squalor which is used to slow down the rate of growth of a settlement and also makes it harder to keep order
As far as I have been able to work out there might be some settlements that have a reduction in squalor and your Governor can have traits that reduce or increase it.
After squalor which affects public order as well a population growth their is overcrowding which just affects population growth, which stops settlements from getting very large populations.
Squalor and overcrowding rates appear to be defined in the desc_cultures.txt file