Why is my tax base differs that much among locations?

I am playing as the Ottomans and I’m confused about how tax base actually works. In Bursa I’m getting a tax base of 10.28, but in İznik it’s only 1.32. Population and control levels aren’t that different, and the is year 1367.

I’m loving the game, but the tax formula feels pretty unclear. I can’t figure out why the gap is so big or what I’m supposed to do to improve the low one.

Can anyone who understands the mechanics well about what actually drives tax base, and how can I raise it explain it to me?

here is the screenshots:

https://ibb.co/ymCNJrpW

https://ibb.co/YGb6x6C