Larian Studios Forums Baldur's Gate III Baldur's Gate III - Mega-threads and Guides Party Size Discussion

We already addressed this: in short, that's not true at all.
First, because if anything more party members would "clean up" the enemies quicker.
Second, because making room for 6 party members doesn't mean you CAN'T still play wioth 4, 2, or even solo if you want (surprise surprise, that's exactly what some people did with the past two Baldur's Gate games).
The mere fact you are splitting exp among less party members automatically address party scalability making smaller parties level up faster.