I mentioned this as a footnote in another thread, but I want to shine a little more spotlight on the topic with it's own dedicated thread.
High productivity or even a strong economy doesn't necessarily mean the capability to produce a large standing army. Factory production of guns and equipment is only part of the equation. The missing element in Civ series is the population factor.
Granted it's a game, but in the real world, population size has always been one of the key factors in determining the size of a standing army, but yet it's importance has been left out in the civ series. I would like to see a city's population slightly diminish every time it produces a military unit, except from planes and machine guns.
Let's take a look at some real world statistics: source: http://en.wikipedia.org/wiki/List_of...mber_of_troops
China - 3,440,00 active troops (#1 army size / #1 population size)
United States - 1,473,900 active troops (#2 army size / #3 population size)
India - 1,414,000 active troops (#3 army size / #2 population size)
Now there are some exceptions to the case like North Korea has the #4 largest army but #47 on population size, but we can also say that N.Korea has taken a huge hit in its GDP/GNP because of it since it has less civilians working. This is why I say that population is one key factor. There are obviously other key factors in determining military troop strength such as government type, leadership ambitions, economy, production capability, etc...
In Civilization 5, I would like to see a more direct factor between population size and troop strength instead of just hammers and money.