Yeah, that's what he explained to me, too. Maybe slightly different details, but the pressures--both for lengthening and for capping the length--are dictated by the booksellers rather than the publishers.

The next question is, why did these publishing demands work out so differently in f/sf than in mystery? Why are mystery novels thinner than f/sf? It's not just that f/sf has *room* for doorstop novels and trilogies; I think the average f/sf book is substantially thicker than the average mystery novel.

I will also note that there are more non-series f&sf novels published than non-series mystery--pretty much every genre mystery novel is assumed to be part of a series. Contrawise, as James pointed out, mysteries are at least always complete stories rather than "whoops, reached the word count, go buy the next two novels to see how it turns out".

(I note that romance novels--outside of the disposable novels like Harlequin--have also gotten physically bigger during the period of the growth of f/sf novels.)