I agree that the population values seem a bit low for what are essentially the world's major commercial/industrial centers, even presuming medieval levels of technology. The population numbers as stated don't seem sufficient to sustain the amount of industry/agriculture going on. If it helps your immersion, you could assume the numbers are to be multiplied by 10, which should be a bit closer to realism (at least on the high side)...
To go back to your initial ask - the ranges that control what population values correspond to which "levels" of city are moddable, (see CoreImprovements.xml, look for the city hubs; the limits are set with XML like this:
Code: xml
- <GameModifier>
- <ModType>LevelBarrier</ModType>
- <Attribute>Population</Attribute>
- <Value>120</Value>
- <CityUpgrade>CityHub3_Conclave</CityUpgrade>
- </GameModifier>
...and the factors that control population growth are also moddable (to various degrees) - but, the problem with modifying them is that there are a number of places in the game that use those as factors in various calculations (e.g. there's a spell that converts population to mana at a 2:1 ratio; if you have higher populations, the spell becomes much much more attractive.) You'd probably need to modify all of these as well - not a trivial undertaking.
