Maximum province level is, ultimately, limited by the availability of resources (primarily food) and technology (i.e. the ability to get food).

The Rjurik are further north so possibly beyond the wheat zone limiting them to poorer harvests than the "civilised" three nations and are I suspect supposed to be slightly less technologically advanced than the three civilised types, so putting a population limit on is certainly easy to explain.

But... trade has a way of balancing things. The City of Anuire for example is L10 despite having no land of its own to harvest (although you could say it trades its technology/culture with Anuire).

Also the sea provides sustenance and the anuireans are supposed to be poor sailors making it hard to argue that the Rjurik aren't better fisherfolk.

In practice the maximum population levels along the Taelshore aren't even close to reached though - levelling up to 4-6 is quite feasible under vanilla rules, and that would vastly increase the population.

I have to admit I prefer the rules that limit growth - RoE had time-based delays via growth points, I've used non-linear scales to increase costs, etc.

I also prefer, to a degree, the concept that province level is the "organised" level of the population not the actual level, so a L1 province and a L3 province may have the same number of "people", but the L1 province people may not answer to the crown, or may lack transport/communication links, etc meaning that they generate less GB/RP - that makes dealing with the "wow we just quadrupled the population in one game year, people grow up fast around here" issue.