Despite this, you can still get away with less units, e.g. once you knocked down the walls you can move your catapult (or whatever) to the next city. Ranged units can hop easily between the two as fast-moving mounted units. Having less units also brings the benefits of more promotions. No-one has said this, but when you forms corps or armies, they all take on the promotion level of the highest unit, so using less units in the early battles is better in the long run.