A general rule of thumb I always used was to take take whatever the in-game population says and multiply it by 10 to get a closer match between the size/complex

A general rule of thumb I always used was to take take whatever the in-game population says and multiply it by 10 to get a closer match between the size/complexity of the city and what the population would more realistically be. If I recall correctly, there was even a few mods that did a similar re-balancing of the population numbers in the first game.