I'm going to respond to your post here.
And as said in the Civ Discussion thread , here is the before-and-after (t)weaking Poland.
The result was 14 cities v 10, but may the sample is not enough.
Also included are some other screenies
1. Kinda nice is the Bavarian variant occuring when the barbs raze Frankfurt after Augsburg flips. (tho kinda sad that didnt last for more that 30 turns

)
3. I hope that Burgundy looks more realistic.
4. Devoted to Edelbroy (that guy who sued for playable and indy Aragon)
Gamewise: Germany is strong if survives, Arabia and Cordoba collapsed in both cases. I think Kiev survived in the first game.
What parameters did you try changing?
I'd like to outline the "story" of Poland. Bear with me -- it's not a bit of history I'm very familiar with. I'm just thinking out loud.
970 - 1140: A relatively large Poland is established. Capitol gets moved to Krakow.
1140 - 1320: Internal divisions fragment Poland. Conflicts with neighbors (especially the Teutonic Order) and attacks by Tartars/Mongols are serious threats.
1320 - 1700: Re-establishment of a large Poland through conquest and union with Lithuania. Peak reached around 1500. Establishment of folwarks. Battles with Teutonic Order, Turks, and Moscow. Golden Age of Poland. Warsaw becomes capitol in 1600.
1700 - 1800: Decline and partition by neighbors, particularly Sweden, Germany (Prussia), Austria and Moscow.
Now, our UHVs are:
Be the most populous country in Europe in 1540AD.
Never lose a city before 1600AD.
Be the Catholic nation with most Faith Points in 1730AD.
These are all later objectives, leaving one rather directionless at the start. That's not necessarily bad -- it's okay to have some Civs like that. The only battles forced upon you are probably with a few Mongols who make it that far. The neighbors aren't likely to pose much of a threat -- Moscow's just too far away, and the Swedish AI certainly isn't going to launch a worthwhile amphibious attack. That leaves Germany and Austria. If they were strengthened it might help. Otherwise there's probably enough space for you to become the largest Civ quite peacefully while building up your faith points.
We can't (typically) model the 1140-1320 period of division very well. We can do external shocks (typically barbarians or new civs), but this was mostly an internal affair. Fine, but if Poland can reach the historically appropriate size by 1150 (which it basically can, starting out with a flipped city and three settlers), then it's just going to keep growing. The "Never lose a city" condition is a useful check on the human. If we spawn some Tartars near the eastern border with Poland, that could force you not to over-expand. Do the current Mongols ever get close enough to really threaten Poland?
Possible ideas:
Move Polish start to 1320ish.
Use RFCEBalance to weaken Poland generically.
Start Poland with less tech/troops.
Increase Barbarian pressure
Strengthen neighbors.