Cool! I hadn't seen that before and it didn't turn up in my searches.
While Masao had gone on about how cramped the ship is and how the decks weren't often at full height, that diagram seems to show near-full deck height for the main/red deck - unless the figures in the diagram are children. :P
This in turn leaves quite a bit of space in the upper part of the saucer for fuel and water tanks, various machinery, and ad-hoc stowage - there's arguably even enough space for a Jeffries tube crawlway up there to access it all. Likewise much of the lower/blue deck is apparently unallocated and has lots of space for extra stuff; and I'm thinking the computer core would fit nicely in the hull blister that has the navigational deflector in it, or up in the suitably-labelled "machinery" space in the upper/green deck.
Now that we know this layout, I'm honestly thinking there's a big Serenity-from-Firefly vibe about this ship. Not everything THERE was explained either, but they had characterized most fo the living space on the ship and it has pretty much the same sort of space, plus a very large cargo bay. If Starfleet is more advanced than that, I can believe the Archer class has more than enough space for all the extra things to make it go.