Some very famous Indie games run best (and sometimes ONLY) with a GM+2players (or with three players and no GM): just to cite some names:
– Dogs in the Vineyard
– Polaris
– Sorcerer
– Trollbabe
– Shooting the Moon
In general, most of the games that can’t be played like this are the “characters party” games, the ones where a group of characters have the same adventure and they are differentiated by their specialized roles: to allow “parties” of 5-up gamers the characters are iperspecialized to the point where you need at least 3-4 of them. Or there are relationship rules to keep them together that need that number of characters.
Even in these PbtA times, there are a lot of games that don’t share these features.