La réponse est donnée par Charles Stross dans un papier publié sur le net. C'est ici.

Petit extrait :

"Anyway. I began selling novels (in 2001-02) just as the trend for longer novels peaked. I'm actually writing shorter books than my earlier ones — my last two finished manuscripts ran to 102,000 and 107,000 words respectively, whereas my first three SF novels ran to 118,000, 138,000 and 145,000 words each. (On the other hand, I'm not necessarily writing less. Two bloated 150,000 word behemoths take nearly as long to write as three relatively slim 100,000 word novels, if you've got your future projects planned out well in advance.)

There's just one outstanding problem with this Just So tale of publishing folk. We who read SF/F may have been trained to expect longer books by the grocery distributors, but why haven't mysteries grown the same way? It turns out that the average mystery is much the same length that it ever was. There are exceptions, but they're obvious as such — you don't regularly see 400 or 500 page mysteries on the shelves.

I would hypothesize that mysteries didn't succumb to the selection pressure for longer books because there's a countervailing force at work — the reader's ability to keep track of multiple characters and plot threads. If you want to bulk up an SF or fantasy novel, the easy (and lazy) way to do it is to add viewpoint characters and plot threads, small stories interleaved within the larger story that shed light on it. But it's hard to do that if what you're trying to hand the reader is a comprehensive set of clues to a fixed scenario, without burying them in a midden of red herrings. Which leaves stylistic efflorecense; but a gritty, relatively terse style that has been de rigeur in mystery since Raymond Chandler and Dashiel Hammett — there are exceptions, but florid verbosity is generally frowned upon.
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