Maybe we should add something like these lines to cap city at level 4 to have growth rate about 0 or less if not set tax to low tax rate.
Code:
proconsuls_palace requires factions { barbarian, carthaginian, eastern, egyptian, greek, roman, }
{
capability
{
population_growth_bonus bonus -4 and not hidden_resource huge_city
population_health_bonus bonus -4 and not hidden_resource huge_city
and
Code:
imperial_palace requires factions { carthaginian, eastern, egyptian, greek, roman, } and hidden_resource huge_city
{
capability
{
population_health_bonus bonus -4 and not hidden_resource huge_city
Then you will no need to fear that city would grow beyond large city if you not force them to be(which not so wise idea to do).
Do health bonuses effect on chance to get random plague in cities? If yes then would be good as overcrowd city should have a load of chance to received plague event