There's no compelling reason for the particular size that applies outside the story I initially had in mind. I was looking for a way to construct a contiguous ISO that housed in a single, gravitationally-stable structure the entirety of an S:4 archailect. I had a long look at the "what physical size is the computronium and supporting material of an S:4 mind" and started doing math from there.
Although this does require a stupid amount of matter, the same can be said of every S:4 or S:5 mind in the entire setting. This ISO was to be distinguished as a (possibly insane) S:4 entity that absolutely refused to depend on wormholes, spacetime metric distortions, and existence as physically separate parts.
However, outside the context of that story, there's no reason for the size. If someone is developing it as a world that modosophonts live on, a local transapient isn't strictly necessary.
Although this does require a stupid amount of matter, the same can be said of every S:4 or S:5 mind in the entire setting. This ISO was to be distinguished as a (possibly insane) S:4 entity that absolutely refused to depend on wormholes, spacetime metric distortions, and existence as physically separate parts.
However, outside the context of that story, there's no reason for the size. If someone is developing it as a world that modosophonts live on, a local transapient isn't strictly necessary.