I co-authored a book published by Wrox Press a few years ago. Editors specifically tell you to pad as much as you can - the fatter a technical book, the better it sells apparently.
Now my default stance is, when in doubt, buy the book that's under 300 pages, since if you can't cover you're points in less than 300 pages you're either trying to say way too much or your book is mostly fluff and filler.
Also I imagine the ubiquity of fast internet connections and Google has made the 1200 page "complete reference to everything" type of books more or less obsolete. More and more books these days are "focused expert knowledge in one particular area" which is a lot more suited for the 150-250 page format.
I think Pragmatic Programmers has a better approach at quantity vs quality.