As an author, I can say that much of the length of my books/courses comes down to experience in the classroom from the previous teach/course or from feedback I get from students. A contrived example:
1) For product release 2.0 in 2000, I create a 2-day course and write a companion 300-pg book. I teach said course and students say, "Dang... not what I hoped for. It's too much intro and not enough stuff that I can use to accomplish"
2) Marketing/sales team comes to me and says, "We need more beef. The feedback is that people have trouble installing and configuring and, as a result, the rest of the book/course is tough. Also, we need to highlight these new features"
3) Version 3.0 of said software comes out and, to accommodate everything and everyone, the course becomes a 3-day adventure and the book is now 400 pages. The feedback this time is "Okay, now we're seeing trouble at the basics - we need more time spent on x and y"
4) Version 4.0 comes with a 4-day course and a 500-page book. The sales team pushes back and says, "No way we can sell a 4-day course. Make it 3-days. And by the way: don't remove any content!" Yikes - that's impossible. The compromise then is that I keep high-level content (overviews, introductions) to sections and move the content from the course to the book. I then have to expand the book by 100 pages (it's now a 600 page book).
5) Version 5.0 comes out in 2010 and it's a total re-design of the software. There are also completely new features that are very complex. Sales says, "Great job on the last course - don't change anything, just add the new features!" Hello 800 page book!
6) It's 2013 and now every pubs manager knows this cycle backwards and forwards. A new tech comes out and the pubs folks go ahead and tell the author, "Don't bother writing a succinct intro - go ahead and flesh it out. It needs to be at least 500 pages or else people are going to complain that it doesn't have enough meat and weight."
"But Scott, this is your problem. A better author could show in 200 pages what you took 800 pages to show!"
There's simply a minimum amount of time that it takes to teach a topic. It's all about "scope" - if the scope of the book is to teach you the basics to a programming language, it just can't be done in 200 pages for any sufficiently complex language. I think that the way to approach this is to (a) design design an actual in-person training course in which real human students will sit in a room with you, and your job is then to teach them a skill that they will have to take back to their job (which paid handsomely to send them). Given a 6.5 hour training day (9:00-4:30 with breaks, lunch = ~6.5 hours of training time), how many days of training is your course? A given day is probably 150-300 pages in a book. If you try to "Teach Java in 6.5 hours", it just doesn't work. You might need 3-5 days to learn the true basics of the language in such a way that you can "think in Java" (which is the basic starting point). Bam - 1000+ page book.