It's interesting that almost half of the plastic enclosure is empty. Could this be to allow for higher-capacity cartridges without having to use higher-density storage?
You see this on the PS Vita as well: the memory cards, which are meant to be changed rarely if ever, are roughly the size of a microSD card while the game cartridges are probably double the width and height even though the storage capacity of the carts tend to be much less (2-4GB) than the memory cards (8-64 GB).