The second enclave isn't a cavern — it's a VR cube the size of a continent, and almost all of it is public space: parks, museums, stadiums, restaurants. Zoom out from your front door and watch home become a rounding error (§24).
◆ In-world canon · §24New Ganymede is modelled as a cube with the same volume as the moon Ganymede — about 4,246 km per side, roughly New York to Los Angeles along every edge. Total volume is ~76.6 billion km³. Private homes — apartments and their teleporter decks and sim rooms — add up to only about 32,000 km³, or 0.000042% of the cube. But this is a VR world, so the rest is not solid: it's overwhelmingly public and "third" space — parks, museums, stadiums, plazas, restaurants and the instantiated environments of the sim catalogue — exactly as New Tripoli's downtown core (§5) is arts and entertainment rather than housing. A trivial remainder is actual compute/maintenance substrate, which a couple of characters note can be grown or shrunk as needed; the world never dwells on "server rooms."
The lived-in details are deliberately generous: a main floor of ~3,500 sq ft with 20-ft ceilings, two flanking teleporter decks (~12 people each), opening onto corridors 10 m wide and 35 m tall, with 4-m-tall doorways — about four times the width and fifteen times the height of the Enterprise-D's, and those corridors stack floor over floor. It feels palatial, and that's the truth of the place: your home is a small private anchor inside a continent-sized commons. The reveal is the opposite of claustrophobic — almost all of New Ganymede is somewhere to go.
This is the ETI's answer to a problem New Tripoli created: residents drifting into private sims and away from each other. New Ganymede builds teleporters into every home by default, making spontaneous gathering the path of least resistance again. Pairs with the Layer Stack and the Time Dilation Instrument (the cube runs at 100×).