Size

http://grognardia.blogspot.com/2009/03/size-does-matter.html

From the comments, by Lord Kilgore: "I honestly believe (and have since the days of the original Unearthed Arcana and the Dungeoneer's Survival Guide) that there is an inverse relationship between page count and potential for easy fun.

Maybe the quicker references to the rules speed things up and make it seem like more is happening. Maybe quicker combat keeps everything flowing. Maybe the DM has to put a little more into it because he can't just read off a table. Maybe the players do too. Maybe you don't take it quite so seriously because it's "just a simple version of the game."

Or, maybe, it's all of that and then some."

I don't disagree, to a degree.  And hour for hour, I probably had a lot more fun simply playing the various setting iterations of "cops and robbers" or "cowboys and injuns" or "the Scooby gang and the monster" or whatever other game we played as kids that had no rules at all whatsoever.  Not that I'm interested in doing that again (even my kids are too old for that kind of thing; I'll have to wait until I can chase grand-kids around the playground, I guess) so I think there's a limit to how far the rule Lord Kilgore refers to goes.  But just for the heckuvit (and for bragging rights) let's make a few comparisons.  Keep in mind; this makes no concession to the presence, absence, or size of artwork and white space, or even the size of the pages themselves.  A word count would be better.  But, what can you do?  This gives a bit of an advantage to the free supplements put together by indie guys on the internet, who didn't pay for any art (although some of them include public domain artwork a fair bit) but by that point, where in the region where the rule about diminishing page count stops having any real meaning, in my opinion.

And I'm not sure how to count up 4e, because of the weird release schedule, I was never really sure which books I would need to look up page count for.  Do you need the PHB 2 or 3 to play?  I dunno.  So I didn't bother counting it at all, and just left it off.

This is also the "core" rules only.  Obviously, most of these lines had a great deal more material available, although it was specifically called out as optional.  This is the stuff that you absolutely need to play.  I fudged that call just a bit too: Pathfinder is generally considered as requiring at least the 1st Bestiary to be a complete game, so I added it too.  This is what I've got:

5th Edition: 992 pages
3.5 Edition: 960 pages
Pathfinder: 896 pages
AD&D (1e): 470 pages
Rules Compendium: 304 pages
OD&D (LBBs plus the 4 supplements): 183 pages
Labyrinth Lord (Advanced; free no art version): 160 pages
Swords & Wizardry: 146 pages
Labyrinth Lord (Basic; free no art version): 140 pages
Moldvay Basic/Expert: 128 pages
Swords & Wizardry White Box: 72 pages
OD&D (LBBS only): 56 pages
Microlite74: Extended: 32 pages
Microlite74: Standard: 28 pages
DARK•HERITAGE m20: 28 pages
Microlite74: Basic: 20 pages
Microlite Purest Essence: 16 pages
Microlite (Original): 3 pages (granted, if you don't already know how to play, you're probably hosed with this one, though.)

As an aside; I sometimes forget, since not everyone has been involved in messageboard and other D&D discussions over the last ten years or so like I have.  Here's a list of common abbreviations:

• OD&D: Original D&D; the 1974 (and later reprintings) of the first version of the rules.
• LBBs: Little Brown Books; which were actually white in most printings.  The actual rulebooks contained in the OD&D boxed set.  This is done, usually, to distinguish between the basic OD&D rules, and the OD&D rules containing one or more of the four supplements that were released.
• AD&D: Advanced Dungeons & Dragons; could refer to either the first edition (1e) or second (2e.)  Occasionally you'll here references to 1.5e or 2.5e; this refers to 1e with the implementation of the Unearthed Arcana (UA) book; 2.5 refers to the implementation of Skills and Powers.
• BD&D: Basic D&D.  Technically, it should probably refer to the Holmes set, since the Moldvay and Mentzer have their own abbreviations (and go beyond Basic) but in practice, BD&D is used as a catch-all for almost all of the D&D games that were not OD&D or AD&D until the release of the RC (see below.)
• B/X: Basic/Expert: referring to the version of the game that game in two separate boxes.  Basic was written by Moldvay and Expert was written by Zeb Cook; but the entire version, as well as being called B/X, is often simply called Moldvay.
• BECMI: The progression of boxed sets as written by Frank Mentzer, with the Larry Elmore covers.  The acronym refers to the various boxes in the series: Basic, Expert, Companion, Master, and Immortal.  Sometimes also simply called Mentzer.
• RC: Rules Compendium; the book version of BECMI after it was revised and collated.
• With the collapse of TSR, the D&D line and the AD&D line were collapsed into a single game again, but the edition numbering followed that of AD&D.  So when you see 3e, what that usually refers to is D&D 3rd Edition, which came out in 2000.
• 3.5 is the revision to 3e that came out in 2003.
• 4e is the 4th edition.  
• 4.5 is a nickname sometimes given to D&D Essentials, which was a restructuring and revision and reprinting of 4e.
• 5e is the newest (and current) edition of the game, which was released in 2014.
• PF is Pathfinder, and since it was specifically meant to be a revision of the 3.5 SRD that stayed more faithful to the 3.5 game than 4e was obviously going to be, many PF fans actually consider PF to be more faithful to what D&D is than 4e ever was, in spite of the fact that it obviously lacks the brand name officially.
• OSRIC: Old School Reference and Index Compilation: the first "retroclone" which used the SRD and OGL to recreate a facsimile of 1e and managed to get away with it.  Spawned, indirectly, the entirety of the OSR
• SRD: System Reference Document: almost all of the rules of 3e (and later, 3.5) and made them freely available for anyone to use or work with under the auspices of the OGL.
• OGL: Open Gaming License: the license that allows anyone to use the SRD to develop their own material for the game.
• OSR: Old School Revival (or Renaissance, or Revolution): products that are compatible with older editions of the game, thanks to the presence of retroclones like OSRIC, S&W, or LL.  As the movement has evolved, to a great degree, it's developed now it's own games that are like evolutionary to those older games, rather than merely being faithful(ish) reproductions of them, like ACKS or LFP.
• S&W: Swords & Wizardry; a retroclone specifically of OD&D.
• LL: Labyrinth Lord; originally meant to be a clone of B/X, but has evolved an AD&D variant as well since then.
• ACKS: Adventurer Conqueror King System: an evolutionary game that expands on the concept of BECMI, where as you level up and the genre changes, you actually do different things rather than dungeon-crawl harder dungeons with Orcus in the room instead of an orc.
• LFP: Lamentations of the Flame Princess: another evolutionary OSR themed game with a decidedly "grindhouse" vibe; gratuitousness and maybe that Ron Edwards "phantasmagoric" except this time turned very, very wrong, feel to it.