The reason why Egypt can mass-produce armies unlike any other faction in the game, is that 2 of the 3 cities along the river Nile have the grain resource. Not only does having the resource boost their population growth, they are also trading grain with each other which causes even greater boost. Hence those cities are growing super-fast, so that the faction can just keep pumping out units without depleting it's population. It also helps that egypt is insanely rich (much thanks to the same grain-trade along the Nile).