The game shipped with way too many medium sized maps, Guardian is the only map I'd call small that came on the disc. There were only two big levels on the disc too, Valhalla and Sandtrap. So yeah, Halo 3 definitely needs more big and small maps, there's way too many medium sized maps.
The Heroic Map pack included Foundry, which I'd say is small. Standoff is another medium map, but probably the best medium map. Rat's Nest is big and really hard to get around on, definitely the worst in the Heroic Map Pack. Ghost Town looks larger than I originally thought after seeing Bungie's Weekly Update on their site today, the screen shots they released yesterday only show one side of the map. Cotton Ball is supposedly Sidewinder (from Halo 1), so that'll be a **** big map, and the other map, Moonlight Sonata is supposedly Lockout, another small map.