Hey Ephycis!

The values which affect the population slots on the planets are defined in the SimulationDescriptor ClassPlanet:

RawMaximumPopulation is the "potential maximum" : given by size and type

BaseMaximumPopulation is the "improved maximum" : RawMaximumPopulation, but it's affected by improvements, anomalies, etc.

MaximumPopulation is the "final maximum" : BaseMaximumPopulation, but it's affected by System upgrades, Vodyani ark level, etc. It's the actual, final value.

You'll have to look into the game files to see what affects each Property, and override the SimulationDescriptors' Modifiers with your new increased values.