![Image](http://www.ditl.org/Vanfolder/PhotonStorage.jpg)
That's a basketball court, one deck high, with six hundred photon torpedoes stacked on it. As you can see, there is plenty of space to get in and out and perform maintenance on them or whatever, too.
Whatever the reason is that the GCS only carries 250 torpedoes, it's NOT a lack of available volume.