I have tried this very thing 4 times, not by intent, but just through the flow of the game. Eacj ofrce I mustered was larger than the previous one. The results were the same each time. The last time, I was playing C3C Middle Ages. I mustered a force of 5 (!) Armies full of elite troops. I also had 15 Knights, and 9 Crusaders. I was full of myself, and feeling my manhood! I headed for Jerusalem with 2 Relics. After many turns, I made my way through Byzantine territory, crossed the border into Abbasid lands, and my force was engulfed within 15 turns. I put my armies on a hill, to attempt to get them to defend long enough to heal. Alas, his assassins wiped out all my supporting cast, and his Ansar warriors did for my armies. I reaped a great slaughter of the foe. I destroyed over a hundred units, but it was worse than a pyrrhic victory. To add insult to injury, by the time it was over, most of my neighbors, including my allies, had declared war on me, and I went down to ignominious defeat.
This has happened every time I've marshalled a large army, because every time I've been at peace long enough to build such a force, the AI has been, too, I find that if I don't get and keep them fighting, eventually, they come at me with a larger force than I can possibly build. Fortunately, I'm a skilled defender, and I whittle them down, then eventually reverse the balance of forces. Usually....