Except that's not how it works with DnD system. Action economy, ability to maintain concentration spells, etc aren't determined by level. It will be either too easy or too hard for a smaller party, because of how DnD levels work.
Pardon the stupid question ... But why is that a bad thing?
If you look at this outside regular easy/normal/hard options ... And imagine difficiulty setting as they did it for Pathfinder ...
Party size could easily be part of that settings.
Because players shouldn't be penalized for playing with a smaller party if it's uncomfortable for them to manage many characters or if they don't have that many friends to play multiplayer. Also Pathfinder is an interesting case actually, considering that the game is extremely easy once you get to midgame even on higher difficulties. Summons make tanks irrelevent, mythic powers make elemental resistances and immunities irrelevent, fogs stack and can be spammed, etc. Smaller parties can easily beat pathfinder as long as you have a summoner and a blaster/controller with very basic min/maxing simply because of how the system works.