Ah, but did you notice that ciries with 1 population have two squares being worked, one for each food point it took to create the settler.
Obviously, one food point goes to the Urban population - this is what the number for the city refers to. The other point goes to the rural population, that remains constent regardless of the size of the city.
Ok there is some problems with this theory when culture expands, but i'm sticking to it!