After a manuscript contains certain number of words, the effort to add more to increases. A 200 page book takes much more than twice the effort of a 100 pages book. Anyone can write a mediocre short story, but you cannot write a some exercise.
On the other hand, a book that is twice as long easily delivers less than twice the value to the reader. You can fix higher margins on shorter books. It is therefore convenient to publish shorter books.
For a long time, technical books have been predominantly 500 pages long monsters. Are there advantages in terms of storage, shipping?
With eBooks, some publishers have starte offering much shorter titles. That is what A book Apart started doing. Now O’Reilly is doing it too. Maybe buying an eBooks has got more of an impulse buy character, and so lowering the price below a critical amount has got an immense repercussion on sales volume.