"This sort of size is necessary to support a tablet which isn't hopelessly tethered to a PC."
Are you implying that iPad is hopelessly tethered to a PC? Have you used an iPad?
As of iOS 5, there are very few reasons that one would need to be "hopelessly tethered to a PC" in order to use their device. Backups, music and app purchases, and contact and calendar syncing are all completely untethered.
As for restoring firmware, in reality that's something something that an average user does extremely rarely, if at all. iOS has a "Reset All Content and Settings" feature which deletes everything off the device and reboots it into a completely fresh state. If somehow, an average computing user like my grandmother has managed to get her iPad to the point where it truly needs a restore, any Apple Store would be more than happy to assist her. In that scenario, what sense does it make to carry around a copy of the iOS 6 firmware on the tablet?