Robert Silverberg once said that the novella is the best length for a science fiction story:
I have said the novella is the perfect form for science fiction in the introduction to practically every collection of my novellas, and I do believe it. The novella allows the detailed working out of a complex science-fictional idea, the portrayal of a culture, the complexities of the character who is enmeshed within that culture. The exploration that I think is at the heart of a good science-fiction story can be done in great detail. At the same time, you don’t have the exhausting and sometimes stultifying process of spinning the thing out to book length. In modern science-fiction publishing, very big novels are expected — for that matter, whole trilogies are expected. In the novella, you can move around within the 80 or 90 manuscript pages and achieve quite a lot. I believe Edgar Allan Poe’s old dictum: one thing happens in a short story. Everything that happens in a short story should depend on that one thing. In a novella, two or three things can happen (or five or six sometimes).
Around 18 months ago, I ranted briefly on the bloat of science fiction. Now, a new discussion talking about the same thing has a brutally honest, by-the-numbers analysis by Charles Stross, who summarizes:
Until the book publishers figure out how to package collections of novellas and pay the authors pro-rata sums competitive with what they’d get for a novel, novellas are going to remain the dumping-ground for failed short novel ideas and special exhibition projects. And they’re going to be in short supply compared to the (much more lucrative) novels.
Sad, but true, I fear.