Disease and starvation follow large armies. It isn't so much as they couldn't summon that many people (which would be a stretch if they could) so much as it would be impossible to wage war, you wouldn't be able to siege due to not being able to feed your own army, and controlling a force that large in battle would be impossible without a massive command structure and radio (could replace radio with wizards).

Someone wanting to field that larger force would want the war finished in a couple of months, and push the enemy over with a show of force