I'm of a mixed mind on this.
Hand #1 (one hand): I don't have the time/patience/inclination to wade through 400+ pages for a game. I mean, it's something I do for fun, it isn't a lifestyle. I don't really need/want that many rules or settings detail.
Hand #2 (other hand): It depends on what it is, I guess, and what purpose it serves. Conan d20 weighs in at 356 pages, ~50 of which is pure setting. It does what the PHB, DMG, and MM do (provide a complete set of classes, magic system, monster, all the rules, etc..) plus 50 pages of setting material. That's actually a pretty good deal, and would have been an excellent deal if it would have been $40 instead of $50. Sure, there are supplements, but if you want to game in Koth, Ophir, Aquillonia, etc..., that book, dice, and pencil & paper are all you need for years of gaming.
Hand #3 (other other hand): I look at things like The New Argonauts. 64 pages. Fun, solid, very playable game. Sure you may need to pull in a bit from the SRD to round it out, and you may run out of options in the book in short order if you go at it hard, but if you're a competent DM, you could probably extend it yourself with less effort than trudging through 500+ pages of something else.
Hand #4 (other other other hand): Gamma World 1e. 56 pages. Tons of fun. Enough said.
Hand #5 (other other other other hand): Hackmaster. Dude. **Eight* 128 page monster books, plus a 9th? Seriously? Wouldn't a pair of 256 page books been enough? I mean I like HM, Kenzerco, Jolly and the guys, but c'mon!