Population does not affect tax income.
http://www.paradoxian.org/eu3wiki/Population
It should be mostly accurate for DW - the IN 3.1 part.
Population growth:
Capital +1%
Manufactory +1%
National focus +2%
Foreign national focus -1%
Nomad neighbour -1%
COT +1%
COT level +1%/100
Under siege -5%
Looted -5%
Scorched earth -20%
Tolerance +/-1%
Forced conversion (modifier - from event or decision) -15%
Plague (modifier) -20%
Medical evolution (modifier) +10%
Strategic defenses (modifier) +5%
Religious migration (modifier) +10%
Settlement policy (modifier) +10%
Wokou raids (modifier) -5%
Wokou slaves return (modifier) +5%
Support nomad raids (modifier) +3%
Allow nomad raids (modifier) +1%