Honestly, there's no reason the postmaster can't hold more, or that Tess can't either, especially on current gen. All they'd have to do is add another page to the Eververse store, which seems easy enough, since in the app the store is set up exactly like that. No doubt we'll get something telling us it's a memory issue on last gen, which, while that would be plausible, other games with much more content (and some that are actually real MMOs, instead of Destiny's MO system) have put more on the old consoles just fine. Ultimately, it just comes down to how the program is written. It seems there's bits of unnecessary code that are keeping the file size up (which is what they said was keeping us from getting more vault space), but this has yet to actually be proven.