Leaving aside AD&D itself, which is kind of a special case, I'm trying to remember the first roleplaying game rulebook I ever saw that was longer than 64 full-size pages. I'm hard pressed to remember what it was. Most of TSR's games up through the early 80s were all 64 pages or less in length, some significantly so. I was positively shocked recently when I went back and checked the length of Gamma World's first edition -- 45 pages, if you exclude all the perforated reference pages at the back of the book. If you expand the page count a little further to 96 pages, I think that covers even more.
Now, I know there had to be longer games back then -- Chivalry & Sorcery comes immediately to mind -- but I never played any of them. The games I preferred could have their rules all crammed into a single book I could carry around with me should I ever need to look them up (not that I did, given how simple they were). To this day, that remains my ideal, which is why I find retro-clones like Swords & Wizardry and Labyrinth Lord well-nigh perfect. They hit that sweet spot for me between compendiousness and completeness that I think is an another essential element of the old school.
How long was the first edition of Champions? I want to say that it was 64 pages or under in length too, because I know it used to be short enough that I could actually play the damned thing rather than cower in fear at its monstrous size.