Population Growth is not set by a file, but by variables such as taxes (the lower, the more people are attracted), recruiting, farming levels, and other factors.

Improving farms gives a population bonus of +1 per level. It is also one of the few improvements that is irreversible.

Putting a general in a city then enslaving when conquering another city is also a quick population boost- half of the city's inhabitants are divided among those of your cities with a general in residence.

You can also modify the edb (export_desc_building) file to add population bonuses by certain buildings. I modded Walls to give a Law bonus. I justify this by having a general remind the people that those walls keep them in as well as keeping bad people out. They got the hint and I got my bonus (roleplay required)

As with any modding, make a back-up before you touch anything!

Thank you for your help. It seems of the many versions of RTW out there most seem broken in one respect or another. The information available on this site has allowed me to learn how to adjust the game for more realism. Game designers seem torn between fantacy and reality.
I now have three versions set up according to my ideas of how the game should play
connorsgrandad.

I also modified my game for more historical accuracy, mainly increasing the population of Germania and Gaul. Roman historians claim those forests were teeming with barbarians, yet in the game, you can barely scare up a spear warband in most cities.

I also allowed Germans and Gauls to capture siege engine shops to produce siege engines. They cannot construct the buildings themselves, but those barbarians were very clever about adapting other's inventions to their own use. Civilis (a Germanic chieftain despite his Romanized name) is even claimed to have constructed siege towers for his assault on Vetera.

But I do love my fantasy, so you can hire Vikings from port cities even though they came about seven hundred years after the game finishes...