Cities that are close in distance can't really Share resources.. a tile can only be worked by one city
However, multiple small cities is useful because of
1. no city maintenance
2. no courthouse needed to work nearby tiles
3. no 'maintenance'
4. in the demo a city starts at a population of 2 at least
My best setups are with the Romans where every city has at least one Grassland/Fish and one Forest/Ironhill
These mini cities can pump out settlers and send them to a major city
By finishing the Hanging Gardens in Rome when it hits 20 pop, I am able to get 2 cities to pop 31 by the end of the game (and could probably have 3, because I usually stop making settlers at that point)
I think the "mass increase" in population is the primary way to go (even if you don't do it with lots of little cities)
In a Big city, your population unit can get about 1 resource and 2-3 trade each... and they get the bonuses of any buildings/Wonders you built in that city
so comparing one city of pop 30 (and 13 supporting of pop 2) to 14 of pop 4 you are going to do better with the former in terms of trade and production primarily due to buildings... It does cost a significant amount of production for the Settlers (Ghandi's 1/2 price Settlers) and you need republic to do it so that it doesn't cost to much population
Take all the boosting buildings that the major city has, and (for a minor city) consider whether it would be cheaper to build settlers to move to the major city OR to build the building here
Assuming Minor city = pop P
Cost of Settlers= 20
Cost of "booster" buildings= N
You can either build the Settler, OR build the Booster buildings
The booster buildings increase the output of the whole small city P* booster benefit; cost= N
The Settler increases the output of only one 'pop unit' 1* booster benefit; cost= 20
Once a city's pop P > N/20
then it starts making sense to build buildings and grow that city instead of siphoning it off to the major city (alter numbers some based on the yield that tiles actually give in this city)
Given that Booster buildings would at Least include Library+University OR Market+Bank (and Temple, Cathedral, and Factory) the
For a Non Republic the cost is not only 20 resources, but ?? food (the amount needed to grow a second new pop that is lost) and since Food cannot be generated by a super large city (only trade and hammers) the Food cost is significant.
This means that Code of Laws is a key tech to go for early game (it is the first government available and a good one for early game)
The only potential down side is that
1. In the demo, and possibly in the game, there is a limit as to big a city can get with settlers or other artificial pop boosts like Hanging Gardens or Irrigation first; a city can only get to 31 (it may be able to Naturally grow afterwards, but that is VERY slow)
2. Once a mega city gets beyond about 36 or so, extra population wouldn't yield that much (extra pop adds 1 resource and about 0-1 trade at that point)
Because of the above, you can't just have one megacity, you want to have multiple ones, and every city that become a mega city shouldn't have been producing settlers, because that is a waste.
Probably you can get to between 2 and 5 mega cities in a normal game depending on difficulty level and land that you have (was able to start on a third on the chieftan demo) . So it might help to have them chosen before you start mass producing mega cities. (so the mega cities can build booster buildings/units/wonders)
Of course as the game gets later, a better use of those hammers instead of building settlers is to turn into units... ether for selling to help you get the Economic win/Rush Wonders+buidings in the mega cities for Cultural win, or do battle for the Domination win... or just to keep you safe from the AIs
Which means that Republic is probably best as an early-mid form of government, that its probably worth going through to set up your mega cities, and then going to Communism/Democracy/Fundamentalism to get the victory you want towards the late game.
This also means that with at least one Megacity being a probably good idea, the ICBM sounds pretty decent.... taking a pop 30-40 city to 1 (or 0) would be incredibly difficult to come back from.
