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A ship that size would surely collapse in on itself
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I don't think so. The Drednaught is bigger than most moons and it's fine. Plus they look pretty technologically advanced so, I think they could've figured it out.
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Moons are significantly smaller than Saturn, moons are materiallistically possible to size up for a ship, but for a ship the size of a gas giant is literally impossible as there are no records of material things being anywhere the size of a gas giant as it is too big to have a core supporting it
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The Hive and Cabal are different, the Hive serve a paracausal power, so it in turns allow them to break the laws of physics as in grimoire that Hive literally eat Light in form of material. The Cabal do not serve a power and therfore cannot break the laws of physics unless it meant creating a ship the size of a planet in a shape of one.
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The leviathan is bigger than Nessus, I think