A bit more data, this time from Tarentum.

Code:
Pop	Exports	Town	DisCity	DisPort	Resources	
15516	1436	Salona	12.5	9	Only Cu
28733	1296	Apollo	10	4.5	Cu, Timb
29777	613	Therm	20	11.5	Cu, Timb
Only thing I would say this time is that Tarentum is my capital, and I've always suspected funny things happen with it.

Oh, and if you notice that Apollonia is ~half the distance than Thermon, and the income is also roughly half, despite having similar populations. So, my current theory is that distance plays a major role, whilst population plays a more minor role.