PI has to reevaluate the development system and redo the numbers. Whenever they add more provinces it disrupts its relation to those of other countries.
First and foremost, development should not be instant, it should take a year per development level. You can literally double a cities development or turn a rural backwater province into a booming metropolis overnight.
Second, by adding some sort of demographic/population system, you can cap province development realistically as cities don't appear in places where no one lives (unless you are 21st century China).
Third, adding a resource stockpile system where wood among other resources are needed to maintain, expand, and keep happy cities would help both the player and AI to prevent overexpanding to avoid economic collapses and setbacks.