When I see a "stack," I vomit and stop the campaign.
It feels like an FPS with respect to the learning curve (ie, it's assumed you're already proficient) and an RTS with respect to the unit spam (lawl logistics, what's that?). And it's freaking boring! I don't want to babysit dozens of units, I [/i]really[i] don't. I got Shogun 2 for my birthday and spent 5 hours total playing it for this very reason.
... But those are more meta reasons. The thematic reason I wanted to make armies more reasonable is that it's one thing to round up some people, arm them and spend a season training them. It's an entirely different thing all together to keep a standing army. Those are bodies no longer crafting, trading, farming, managing, etc. That's why the free upkeep units are labeled "militia," after all!
(Even so, I fully admit it's largely a "me" issue, and I expect exactly 0.3 people will even consider going down this road. )
Note I play Blood, Broads, and Bastards mod because. Yeah. Because.
Solution 1: Swap
So I swapped the unit purchase cost and upkeep costs (except for Mongols and the Crusader rabble group there... maybe for artillery too, can't recall). It was easy... but time consuming, as I did it by hand.
The AI, as we all know, is behavioral instead of analytical. As you all probably guessed, every faction ended up bankrupt in short order. While that did provide for far more satisfying battles for me, by the time my ruler's great-grandkids were coming of age I was bored.
I will note that rebels and uprisings were far more interesting! No more surrounding the rebels with stacks and hitting "Auto-resolve."
Solution 2: Average
I was thinking of averaging the two costs and making them identical, but then I realized all this will do is stave off the same situation. The AI still wants to make the same size army. Save it money on upkeep and it will spend that money on... more units.
Solution 3: Max Out
Maybe I will make purchase and upkeep the same, and the higher of the two values? That way the AI cannot buy a plethora of units so quickly. By then I think the situation may become sufficiently dynamic that other concerns (eg battlefield attrition) come into play... but I feel that's still Solution 2, hinged on a weak hope.
Solution 4: Prioritize
Change AI priorities? Make them a little more buildy so they can sustain a Solution 2/Solution 3 army a bit better (and are less likely to make one)?
Solution 5: Koei-ze
That is, maybe making trade a bit more valuable would help, so that this becomes more like old Koei games (before they made brawlers) of not just expanding your empire but managing it as well.
Not quite sure where to go here, 1 and 2 are insufficient. 3 is more of the same. Changing AI priorities in 4 could help, but I don't want to vampire the uniqueness out of AI factions. And 5 may be like 1-3, just staving off the inevitable (and playing to my other annoyance, a perfected drive toward the latest units instead of the economy to sustain them).