Thats an easy answer but its not an answer to my question.
How was the amount of GovCap determined thats available over the course of the game?
The wise Johan ascended a remote mountain and was handed down the numbers that are just right by some dignified-looking bearded guy? (could also have been a lady in a pond, though

).
More seriously: I am inclined to think the numbers are quite well balanced. In my first 1.30 playthrough as France (going for France's missions, not doing a WC because I have zero interest in that) I was very close to (and intermettently above) my government cap for most of the game and at several points had to take into account the government cap when making strategic decisions - regarding what to conquer, which conquests to state when, when and where to build state houses, court houses and town halls, whether to enhance my capacity or get government reforms quickly, what reforms to take (there are some that increase capacity).
I had the impression that the government capacity was low enough to actually be something worth considering and high enough to be manageable. And I believe that the expansion speed encouraged by the French mission tree is a rather good benchmark for a rate of expansion where the cap should be relevant but manageable.
Should it be possible to expand more than in a run where you just do the French missions? Sure. But in that case, I believe it is appropriate that government capacity puts an actual strain on you.
I think that (as problematic as historical comparisons to EU4's game mechanisms are) for example the historical Ottomans at their peak had grown so much that their government and administration ended up being unable to deal with the strain, i.e. it would in game terms actually be right for them to be "over their capacity".
And on the other hand, if the capacity were to be made, say, 25 percent higher, I could basically have ignored government capacity in that France run. Which I would consider a bad thing. I want there to be things to think about strategically when I am not doing a WC. A change that makes things less "tedious" for someone doing a WC would actually diminish my game experience as a non-WCer.