: In "1001 Nordic Nerds" on Rubicon, Durandal says his brain is
: bigger than "any planet in the galaxy." This can't be true can
: it? How much physical space is he stored in? Okay, it has been stated in
: M1 that rampant AI growth demands planet-sized networks, and at that time
: Durandal shared a Deimos-sized network with two other AIs.
Although Tycho says to "leave the strategizing to those of us with planet-sized brains" on Rise Robot Rise, that's just him trash-talking the player. And perhaps a nod to Marvin, whose suffering a rampant AI might find relatable. "Here I am, brain the size of a planet and they ask me to take you down to the bridge. Call that job satisfaction? 'Cos I don't."
As for an AI's actual size, we are told that Durandal's transfer from the Marathon to Boomer was 64 billion exobytes copied in half an hour or less. Tycho's core was the same size when the Pfhor copied it for their own use. And yet Durandal's primal pattern, apparently stripped down to the bare essentials, fit inside the protagonist's wetware. Wifi and flash storage are clearly quite advanced in the future.
As you say, we are also told that a rampant AI grows exponentially and needs a planetary network to survive. This is contradicted by Durandal's survival for years aboard the Marathon under Bernard Strauss's influence, followed by seventeen years happily cruising the galaxy in a Pfhor ship less than 2km in length. He complained bitterly about living in a box, but he lived. Although rampant Tycho's stability is questionable, he also survived and functioned within the confines of Pfhor ship networks. Evidently Strauss knew or discovered something that James B. Miller didn't, something which Tycho inherited when the S'pht reconstructed him in Durandal's image. No such luck for the Vylae and their fifteen-world Leela infestation.
How human AIs are able to seamlessly interface with the hardware and software of previously uncontacted alien civilizations remains unexplained. Perhaps the answer lies in what the UESC found on the moon...