I don't get it. You make a huge army so the AI doesn't declare war on you, but then you just sit on it instead of steam rolling the AI with it and winning the game? Why?
It's an 8 player game on standard map size, not a 2 player game; an army just big enough to deter your opponent from DOWing you while likely to be enough to eventually capture a few of their cities isn't necessarily going to be decisive enough for those AIs not involved in the war to gain ground over you. There might also be places in the tech tree where defense is stronger than offense, meaning the army just big enough to deter would in event of an attempted offensive war degrade into a bloody stalemate.