Quote Originally Posted by KiOwA
Food imports/exports are currently bugged, the city that is exporting grain is receiving the pop. growth bonus, instead of the city that is importing it. This explains why cities such as carthage and memphis often grow to humongous levels.

If I'm correct I believe it works correctly over land but not by sea. Anyways if a city is trading grain it gets a population boost of 1.5 percent, if it's importing grain from 2 different cities it gets a 3 percent boost. No extra bonuses if it is trading with 3 different cities with grain as 3 percent is the cutoff, at least I'm pretty sure about 3 percent being the cutoff.