More of everything is not really true.
Food, Timber and Metal. On most maps, you can keep yourself capped on pretty much everything with only three cities, as long as you efficiently use production boosters (granaries, etc.) 15 farms produces +450 food. With 3 cities that's +480. If you had any fishing or rares you'd be capped out. Same for timber and metal.
Wealth. 4 cities allows 6 trade routes, and, of course, in team games, you only need one city for a lot of trade routes. More cities means more territory, and that means increased taxation, but if you are capped for wealth from caravans and fishing, then extra territory would go to waste. Taxation is a good reason for expansion though.
Knowledge. Extra cities also enable you to make more universities and to produce knowledge. If you are booming that's great, but usually there will be fighting early on and so having more than three full universities can be a little overkill, particularly if you use the university upgrades.
Oil. This is the biggest reason I end up making more cities. If you have a small territory, then you can be competitive up until Industrial Age. At that point Oil becomes important and requirs you to have a big territory to have access to the oil wells. Also, you can put a refinery in EACH town to give you +33% oil production. If you only had 3 towns, that would only be 2x. But if you had 6 towns, you could be at 3x, plus you'd have easy access to a lot more oil wells.
Rare resources. Having access to good rare resources is an excellent reason for making more cities and controlling space.
Denial of resources. Not to be forgotten is the flipside of the coin to the above - denying resources from your enemy. You can squeeze your opponent out of valuable resources like oil, rares, and fishing lakes (e.g., African Watering Hole). This is a great reason for extra cities. This is the reason the "border push" is so powerful.
Depth. Another major reason to make lots of cities is to create defensive depth to your empire. (Depth used to be one of the Characteristics of the Defense). Empty border cities are great for keeping the enemy far from your capital and your economy. Having deep borders with heavy attrition discourages raiding. It also foces the battles closer to the heart of the enemy and away from you.Wonders. If you like to build wonders, then you are going to want to have a few extra cities, since you can only place one wonder in each city (Egyptians get two). I usually only try to get 3 wonders or so, so this isn't a big deal for me.
[This message has been edited by Out_4_Blood (edited 11-03-2003 @ 01:11 PM).]