(Commenting on Demiansky's proposal of a percentage of prestige to attract the people, and that food follows the people.)
Thats an interesting notion ... I only wish that the food have to come from somewhere ... as opposed to people being fed on prestige alone, and farming not being required. (or rather, avoiding extreme cases where 100,000 people are being fed on the food of 30,000 people)
I DO agree however, that prestige should be percentage oriented, so that if every city somehow has equal prestige, the population will be equally divided among cities. If however you have a city with lots of accumulated prestige/culture, the vast majority of your population will be centered around the one city, with the rest scattered throughout the other cities.
A city with no prestige might remain stuck at Town status indefinitely, unable to grow from low numbers ... where a small prestige city would slowly grow as say 5% of the nations population will reside there, so once 5% is larger than its current population, it will grow to meet that number (through migration or births).
This again, however, addresses the point that Prestige should not be an absolute value of population, but relative proportions of your Nation's population. Therefore I think there should be farming or some-such to raise absolute population ... while you get a certain "rate of immigration" based upon your prestige somehow .
We run into similar problems of absolute growth only being able by immigration, which is why I think absolute growth should be tied to farming/agriculture, and the movement of that growth be directed by Prestige. With this system, perhaps it should be important to keep count of the "wilderness" population to some extent. The wilderness population is able to quickly repopulate itself, but will never grow larger than a certain global value. This is my idea. Although this wilderness effect will be less visible in the grand scheme of things (at least supposedly), as truly gigantic empires will only be enabled by a combination of prestige and farming for absolute AND relative population growth.