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upkeep is the unhappiness you receive per citizen.

I think there are late game doctrines or buildings that can give you happiness for what climate type your city has sectors in.

Ultimately unhappiness does limit practical city size even if you have infinite food.
 
On the test branch they just added a new function where you can sell your excess food if you don't want a city to grow anymore. They did it the best way possible too. You set it to export the food to other cities and if no other city takes it then it gets sold instead.