I play on huge. It adds a lot more depth to the game, as the death ratio remains the same, but it instantly means a lot more men. Now try it with a barbarian faction like germans, where your biggest city has about 3000 men at start, and you'll soon value even the cheapest peasant (as it means 240 of your precious men).

Other than that, it just looks nice to have large armies running around the screen.

There is something bad about it too: the rather weak pathfinding has real trouble handling huge units. In cities this can get especially painful and frustrating.