Stross's blog posts on the series are fascinating reads. The short badly paraphrased tale is that Stross wanted to diversify his publishers but was contractually obligated to give sci-fi books to his first publisher, so he pitched a "fantasy" book. Stross being Stross though, he can't actually write a fantasy novel so it was stealth sci-fi, because of course it was.
He wrote the first book in the 1000+ page "door stop" form common to a lot of "high fantasy" mega-authors like Stephen King, Neal Stephenson, GRRM, Robert Jordan, et al, because Stross wrongly assumed that was the intended form factor of a fantasy novel (because it wasn't his native genre and when you do look at bookshelves they are rather dominated by the mega-books), and the publisher did exactly what JRR Tolkien's publisher did for Lord of the Rings and said it was way too long for a first book in a new series, as that would be expensive if it didn't sell well, and chopped it (somewhat roughly) into three books. (So the combination of the first three books was actually recombination into something more resembling their original manuscript form.)
He wrote the first book in the 1000+ page "door stop" form common to a lot of "high fantasy" mega-authors like Stephen King, Neal Stephenson, GRRM, Robert Jordan, et al, because Stross wrongly assumed that was the intended form factor of a fantasy novel (because it wasn't his native genre and when you do look at bookshelves they are rather dominated by the mega-books), and the publisher did exactly what JRR Tolkien's publisher did for Lord of the Rings and said it was way too long for a first book in a new series, as that would be expensive if it didn't sell well, and chopped it (somewhat roughly) into three books. (So the combination of the first three books was actually recombination into something more resembling their original manuscript form.)