The further you get away from the handful of popular RPGs and into the realm of boutique games, the greater the ratio of buyers who will never actually play the game at the table, and for whom the book is essentially just reading material. I expect publishers are well aware of this. These games tend to be even worse than D&D at presenting the system in a concise and easily referenced manner.None of this demand massive hardcovers. The Red Box set had all of this. If it hadn't half of us wouldn't be here talking about RPGs.
5e could be perfectly playable and well-written and fun to look at in under 100 pages, just as an example.
I think price point creates bloat, just like with video games. Most of the promised 40 or 80 or 160 hours of open world games is busy work to justify a $60 price tag. Likewise, most of the 350 pages of Numenera is filler to justify it's price. Some games don't even hide it with walls of meaningless prose anymore -- I'm looking at you, Alien RPG. And I really like everything about the Alien RPG but that is a 64 page game.