do you mean the income shown on the construction screen and on the campaign map, or the one in the Settlement overview(with all factors of pop groth, public order and income are shown)?
in case your reffering to the first: your army's salary is paid by your cities, to be precise costs for all units and Commanders are summed up and distributed to your settlements according to their population eg If one city holds a third of your population it has to pay a third of the summed up wages. these costs(together with corruption devastation etc) are subtracted from a city's income from Tax, Trade and Farming(and some other minor factors) thus when a city grows it can easily get a worse balance, corruption and devastation may be a dicisive factor aswell.
If you're reffering to the second one I have no idea.
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