There seem to be two distinct elements to this.
On the one hand, the physical size of the book(s) causes real-world issues with transport, storage, and to some degree cost, and the more books you need at the table the worse all of that gets. A full-sized 400+ page volume isn't even all that convenient to read, and takes more effort to consult for any specific rule even with the best index. There's also a real question about whether stuffing everything in a single complete volume is better or worse than splitting into multiple books that are primarily GM- or player-focused - and that goes beyond style preferences into economics, which can in turn impact the success of a system in the long run.
On the other hand, having a huge page count makes a system harder to master at even a basic level, and it's a royal pain at the table if you need to refer to the rules regularly during play rather than (say) your character sheet or GM notes. That can be mitigated considerably if large chunks of the book get out of the way while playing, eg an extensive character generation system that you won't actually look at much past session 0, or lengthy sections on encounter design and session prep stuff the GM will distill down to a few notes for play. That kind of thing I'll give a pass too, especially if the core engine is very concise.
My ideal be something like M&M 3e with separate GM and player core books that have reasonable page counts but still manage to be quite comprehensive so supplements really are optional purchases. They're a little dense to read the first time through and would benefit from a slightly more streamlined system, but enough of the page count is invisible at the table it's within my tolerances. Ultimate Prowlers & Paragons might be an even better fit for me, but doesn't have quite the same level of versatility so the tighter engine is a tradeoff.
But I do make exceptions, like Sentinel Comics' 400+ page brute of a core book - but only because about 80% of that is stuff you'll never look at during play once your character is done, and the player-facing part of the game engine takes up a slim 40 pages even including art and examples. It's also got a pretty good index, which excuses many sins. Would I be happier if there was a separate "game engine" booklet for players? Yes, and that is one of the arguments in favor of buying the starter set - but the engine itself is easily learned and recalled, and your characters sheets do so much of the work for the book that using the core book isn't all that much of a strain. If you really needed 100+ pages of rules at the table the way (say) AD&D does I'd probably feel very different about that, but 40 I can accept.