Responses in this thread convinced me to check out Mothership, which I had sort of dismissed as an OSR version of Alien (I don't know where I got the OSR impression from). Holy crap is that a cool little system in a well laid out thin book. Thanks, thread!
GDW did an incredible job with thin book rule sets for the original Traveller LBBs as well as T2k's 50 total pages combined for both player's and referee's books.
As decades go on, thin books are, perhaps, less meaningful, because so many of the single page games and such rely on accepted standard information and available meta-knowledge about rpgs to help them along. Comprehensive rule sets from the late 70s and early 80s didn't have the same foundation to build upon. Especially for games that had to instruct detailed crunch and worldbuild authentic fluff in a handful of pages.
The Esoterrorists is 162 pages long, but only 78 pages of that are rules (the rest are sample scenarios). Esoterrorists introduced the Gumshoe system, one of the most important RPGs of the last two decades, and with rules < 100 pages.
I guess the big issue here is, if you ask for 100 pages or less, you're essentially excluding most games with an actual setting and/or which include any example adventures, and this creating a slightly odd situation where you're going to be getting some games which are kind of complicated or fiddly or just not very well-designed, but are only giving you the rules, and nothing else, requiring you to have or make up a setting and giving you no guidance on adventure design or the like. Whilst excluding games that are elegantly designed and run quickly and well, but actually have a setting/scenarios.
Like the versions of Fate people keep mentioning? None of those are complete RPGs in the same sense Esoterrorists for example is. They're simply tools to manufacture an RPG with. Black Hack and White Hack are similar. They're not complete RPGs, they're just chopped down versions of D&D, that assume you have a D&D setting and adventures to run with them, and make massive assumptions about familiarity with various D&D tropes, monsters, etc.
Typesetting also comes in as a major issue. For example, Dungeon World clocks in at 410 pages, which makes it sound like some kind of gigantic Pathfinder 2E-esque horror show. But it's because there are hilariously tiny word counts per page, due to a massive typeface and the largest amount of whitespace and margins you've ever seen. I looks like it averages about 220 or so words per page. Whereas 5E D&D 660-ish words per page. If it was formatted like 5E, it would thus be 135 pages long, and so extremely close to your requirements. Hell, the 3.5E PHB averaged over 1000 words per page (1047 to be precise). Thus if Dungeon World was typeset like 3.5E D&D, it'd be 86 pages long and thus under your required length! Interestingly it doesn't make the same assumptions as Black/White hack either re: monsters. I actually didn't realize how big of a difference the layout/typesetting could make though - we're talking the difference between a 410 page book and an 86 page one. Wild.
I'm not saying you're wrong to want under 100 pages, but I do wonder if there's a bit more to your request, like, do you want a complete RPG, i.e. with rules and a setting (not merely a vaguely implied "D&D I guess" setting or the like), or are you happy with just rules? Is it for portability? That would make sense. If it's for simplicity and actually making a genuine "short RPG", then I think the typesetting and what's included as well as the actual rules will make a huge difference, as will how hard the game leans your knowledge of tropes.