Just messed about a bit with my Poland save.
Warsaw has 10 for base tax, Krakow has 12. I have +25 at +3 stab and -10 at -2. Krakow has 15K inhabitants, Warsaw has 11K.
When I change the pop of either adding 100K the tax income, the census one doesn't change. Production obviously does. When I change the stability from 3 to -2 the tax income drops from 13 to 9 for Warsaw and from 16 to 11 for Krakow (both provinces have taxcollectors). Pop doesn't matter, stab does.
When at stab 3 u can draft 13K at Warsaw and 16K at Krakow. Once again I added 100K to each. No change. Still 13 and 16. So pop isn't the factor. When dropping stab from 3 to -2 the numbers decrease to 9 for Warsaw and 11 for Krakow. Stab does very much matter.
When decreasing the base tax from 10 to 5 in Warsaw u can only draft 7000 troops at stab 3, regardless of pop.
I also changed Warsaw to a city with just 900 inhabitants. Results stay the same. At stab 3 13 income and 13 troops can be drafted. Same goes for stab -2.
*Edited to fix some stuff Agelastus mentioned, good man

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