This one is easy: the Covenant learned from their mistakes at Reach. They managed to destroy a number of stations, and either avoid them completely by exiting out of slipspace below their orbit, or "blow past them" like the Assault Carriers did in Halo 2. The ODP's yield, at its highest, is 50 Gigatons, if you ignore the Teraton yield and likely mistake in the Halo Encyclopedia. The Mantle's Approach is 3 times the length of a Forerunner Fortress Class battleship. Go figure.