I think there are several categories, including:
- The core system rules.
- Elaborations on the system rules--examples, options, etc.
- Content that sits on the mechanical boundary--monsters, spells, etc.
- Pure content--setting, most adventures, etc.
- For lack of a better term, "design notes"--the why of the system, only useful to some readers, but highly useful to them.
- Boilerplate, fan fiction, and other "filler".
I'm leaving out art and layout, because while certainly important, those operate somewhat orthogonal to all of the above. (You can have great art and layout with great content. You can also have great art and layout with fan fiction. One of these things is much less valuable than the other.)
Then there are the purposes of the various documents associated with the game: Reference material, teaching the game, inspiration, provide content, etc.
My preference is that the core system be as concise and clear as possible so that the elaborations on it can fit in a reasonable profile and provide plenty of examples for clarity. Most games don't give enough good examples in the rules. More often than not, when someone tries to rectify this, they try the "IBM Technical Manual" approach like Hero System 5E does, where they go all in on tedious examples as a reference, not as inspiration. And of course, examples can clarify a well-written rule that isn't quite perfect, but they can't fix bad rules or even sometimes decent rules poorly written.
Content, whether mechanical or not, should be as long as it needs to be to get the content across. If it isn't mechanical, it can be a little more expressive and artistic, within reason. I prefer that the content not be included in the same document as the rules, for both reference and teaching reasons.
Finally, given my druthers, all first editions of games would be in multiple, soft-bound booklets. Even with PDFs its easier to have multiple docs open. If a "playtested in the wild" 2nd edition wants to be in a well-organized single volume with better binding, I can buy that.