I realize this is self promotion for your textbook company (who's goals I laud! Acessible knowledge is a good thing), but the idea that a text book is "too long" is...odd. I've never been expected to read a textbook cover to cover before, and all the extra content a textbook has makes it a remarkable resource for referencing even after the class has finished. While the basics of knowledge can be disseminated in far fewer pages, the upsides of having extra examples and asides in a textbook pretty easily outweighs the downsides (unless you count pricing).
I agree that examples and asides are useful. What I had in mind is certain chapters in physics or math books that are fully boil down to one equation, but somehow end up stretching the narrative out to fill dozens of pages.
Just the other day I was looking for the statement of Stokes theorem in a mainstream calculus textbook, and fell upon two special formulations, one 'outline' of a derivation, before finally finding the theorem. Even though I knew exactly what I was looking for it took me minutes. What would a student remember from this chapter?
A much better structure would be to state the Stokes' theorem upfront and then give a few examples, and maybe show the derivation, then end the chapter. Short and sweet; something the reader will remember.
I'm guessing the value of concise texts depends upon your learning style. Personally, I like to read beforehand and mull over topics so that I can engage my professor in class, rather than just trying to keep up. This would be extremely helpful to me, because I tend to become frustrated when I see unrealistically long reading assignments, and then I abandon the way that works best for me. I was recently assigned to review three 600-page references before a two-hour lab; when that happens, I stop reading altogether. I know that that's far beyond what would happen with inflated textbook chapters, but it is similar.
My fiancée, on the other hand, does her reading afterwards. She goes to class to see what the professor cares about, and then looks for those areas and reads about them. She probably doesn't mind long texts at all (I'll have to ask her.)
