It's balancing in a game. Basetax really is more than just population, though pop is a big factor when considering city-provinces. Provinces did produce more than one good in real life, and that depended on more than an abstract 'basetax' value.
Large cities can also be full of poor people.
I do not think that Athens needs a tax boost, however. 5-6 is fine, the city (and it's very much a city-province with few agricultural areas) was way past its prime and not a trade hub of any kind.
...and this should be fixed by letting provinces produce multiple things, and modelling Victoria 2 style pops for taxation instead of base tax. Such a change would deepen the economy and generally non war side of the game, which I would see as a great improvement.