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.
In the words of the senior NCO instructor at cadet battalion: “If you ain’t cheating you ain’t trying. And if you got caught you didn’t try hard enough!”