pointofyou
Hero
Looking at stuff I'm poking around in and have handyI think most people agree that the book could be reorganized a bit and there's always room for improvement. As far as the length of the books, I'm sure there's a lot of factors that go into it. No, it's not set in stone but if you look at the shelf of a store selling TTRPGs (not just D&D) most of the books are quite similar size and format. I would assume there's reasons for that, whether it's tradition, what booksellers expect and want to shelve, cost/benefit analysis of more pages it appears they have a target length and format. I don't expect that to change.
The Cypher System Core Rulebook is about 450 pages. It contains player rules and a small bestiary and a pretty good GMing section with advice on setting up a game in a number of genres and settings.
The Zweihänder Core Rulebook is about 670 pages. It contains player rules and a sizable bestiary and lots of information about the presumed setting and a large GMing section with much more in the way of actual GM-facing rules and a few sample scenarios. It is difficult to imagine someone reading this book and not understanding the game's presumptions.
The Haunted West Core Rulebook is almost 800 pages and reminds me of some of my college textbooks. It contains player rules including a lifepath system that takes up more than 125 pages and the GMing section contains sample scenarios and sample places and a small bestiary. In addition to a kinda slim section roughly equivalent to Running The Game there is GMing advice strewn throughout the book and though there are several specific ways to run the game laid out in the book the presumptions of them all are clear and there is no particular favor given to any particular approach.
There are really two points here. One is that there's plenty of room in the market for TRPG books with larger page counts. Someone really into rules-light games could probably provide examples of slimmer books but that really isn't my preference and I'm really not that someone. The second point is that It's possible to provide good GMing advice in a TRPG book no matter the page count.