Civilization 5 population demographics

In Civilization 5, when the game ends and it displays the score screen ranking each civ by things like GDP, literacy, population, and army size, how is the population calculated? It doesn't seem to make any sense. In one game, I'll build tall, and have 1-2 cities size +60 and 2-3 more size +30, and my population at the end will be around 200 million. I just finished a game as Washington in with a conquest victory, in which I owned +90% of the cities, and I only had 170 million population. Most of my cities were around size 20, my original few were size 30-35. I must have owned +100 cities. If the population at the end is just a simple sum of the city size, I should have been far higher than 170 million. So how does it work? If it matters, I'm playing with all the expansions.

I'm mostly interested in Civ 5 here, but I would also like to know how it works in any of the other Civilization games.