One thing that no-one seems to have mentioned so far is this:
Each citizen consumes 2 food (apples)
So if your city has 5 population, and is working 4 farmed plains, and one desert with a trading post, your city will be making:
2 food and 1 production from each of the plains = 10 food and 5 production
1 gold from the desert (I think... vanilla Civ V might produce more gold)
Meaning your city makes 10 food, 5 production, and 1 gold, plus whatever you get for the city tile (which is free). HOWEVER, each citizen also eats two food, so the city is eating 10 food, so (in this hypothetical example where the city tile is not producing ANY food, which never will come up) the city will be displayed as producing 5 production, 1 gold, and NO FOOD.
This is not a huge problem, as it just means that your city will not grow, so long as it has no food. However, a small city produces less resources, and less science, than a large city, so you want a large city.