Good question.
I’m only guessing from casual observation, but it definitely has to do with your city size on a straight line. Some type of base number (assume 10) then add 2 needed food for every pop point. (since cities start with 2 pop, then to go to size 3 is 12 food then size 4 is 14 food, 5 is 16, and so on) Cause the pops don’t ‘require’ any of the food. You can’t starve a city.
That’s just my best guess.