My main experience with small number RPGs is D&D 5e (I‘m sure much of what I’m about to say applies to other editions as well as other TTRPG systems, but I‘ve never played any, so I won’t be talkin about em). Despite D&D‘s reputation as a math-heavy, impenetrable wall of rules, 5e is impressively legible. First, its math is dead simple. If you can add, subtract and halve numbers up to 50, you’re set. There are no formulas, no charts, no calculators needed. The game is designed to be lightweight and quick, and also to be easily improvised.
With a decent understanding of the game's systems, the DM can essentially create any rule they want. You don't need to look in a rulebook to understand how difficult it is to do something; you can just think something along the lines of: it's pretty difficult, they'll need like an 18. The Skills system means that climbing a wall is the same as picking a lock is the same as lying to the cops. The facilitation of improvisation is what I find so fascinating about 5e, and those small numbers certainly help.
Aside from the small numbers stuff, I also love the design language of its spells and abilities, but that's maybe a bit beyond my understanding, lol. I just know it makes it trivially easy to homebrew your own stuff, which is one of my favorite parts of running the game.