descr_settlement_mechanics.xml - in your data folder. Do not simply double click the file - it is an .xml file and Windows will try to open it with Internet Explorer instead where you can't edit it. Right-click on the file and open it with Notepad. Oh yeah, make sure to make a backup of the original altered file somewhere and LABEL THE FOLDER where you keep it.
Anyway, here are the snidgets that affect settlement growth:
There are three types of factors it seems: "SPF," "SOF," and "SIF." I am not sure what they mean, but looking at the contents of the file the first one pertains to population growth (probably "settlement population factor"), the second pertains to public order (probably "settlement order factor"), the third pertains to income (probably "settlement income factor").
You can kick up the numbers there for your campaign, even even out the factors between castles and cities (BC has lowered growth bonuses for castles). Yeah, those were two "evens" in a row.
The following is in the end of the file and gives the numbers needed to upgrade cities and castles. You can toy around with those numbers as well to make it easier to upgrade settlements.
By the way, you can also note the sections pertaining to corruption and devastation which I have long felt were already too high in vanilla and are way buffed in BC. I nerf the coefficient (don't mind my fancy words) factor numbers down in my campaign as I feel corruption based on distance sucks and armies are prone to cause too much devastation even when you don't want them to.