Main source of early game is luxury & strategic resources trading ...
Later in the game (past Economics) trade post spam becomes the main income source for most empires (unless you are Arabia) ...
Both those income sources are directly dependent on the amount of tiles (cities) you have ... (you cannot get 30 luxury copies in 30 tiles, you cannot place 200 trade-posts in 100 tiles)...
No wide empire -> No (need huge area to get large amounts of ) -> No huge army (need lots of to produce and maintain huge army) -> No wide empire (need army to conquer land ) {circular dependence list}
HUGE ARMY = REQUIRES CONQUEST WARS to provide the huge amount of tiles required to generate the huge it consumes ...
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In the vast majority of cases you cannot expand too much without conquest (peacefully - unless you are Austria perhaps) ....The more aggressively you settle early the more likely you are to cause incoming DOWs ..
Oligarchy - does nothing to alleviate the huge & costs required to get a huge army in the first place ...
You don't need a huge army if you play defensively and have Oligarchy and defensive buildings. An attack is not going to get very far if you can kill units in two shots.