While I can figure out the metaverse reasons for the Well World being a planet, (copyrights on Ringworlds, and the overwhelming scale of a Dyson's Sphere throwing off readers) I don't get why a Godlike race capable of having anything they can dream up would settle for their super computer being as small as a planet. For what the Markovians were trying to do, I'd think they'd want something much, much bigger. It just would have been nice if Chalker had toyed with that and given the Markovians a reason of their own for making the Well World so small.
Fridge Logic moment in Jack L. Chalker's Midnight at the Well of Souls. All new arrivals on the Well World are sent through a gateway that drops them off in the territory of one of the (many!) native races, transforming them into that race's shape in the process. This happens to Datham Hain, a Manipulative BastardJerk Ass, and it transforms him into a three-meter insectoid creature. Datham immediately spots other three-meter insectoids, and realizes he's now one of them. All well and good, allowing for the Well World's superabundance of Applied Phlebotinum ... except that, from the first glimpse, he knows that the creatures are three meters long. There are no familiar objects or life forms within his field of view to compare their proportions too, and he can't be judging their size relative to his own, as he's now larger than he was as a human. So why does he instantly guess them to be three meters long, rather than assuming both he and they are human-sized, or — given that they're insects, which have always looked tiny to him in the past — that he's shrunk?