I'm not sure how much population affects taxes but your happiness factor takes a nosedive. By the time you get a large city and be on your way towards huge city, you'll need very large garrison sizes just to barely keep it at 100%.

As for training troops, by the time you get a good enough population, training troops every year still nets you additional population.

Also, all my governors are getting way too many inefficient taxmen vices so the percentage of taxes as part of my income is much less.