Party of 6 does not always mean more ennemies. This is a wrong argument.
All else equal, if you allow parties of 6 right now, the game becomes much easier. You could balance this through split XP as mrfuji suggested, or by adding more enemies / stronger enemies.
XP split doesnt really work for the endgame when the encounters are balanced around a full party at a max lvl. And adding more or stronger enemies sounds easier than it is done. It will require to balance encounters for every difficulty level for different party sizes, which will be be a huge amount of work, considering that we are talking about enemies that are smarter than in other DnD RPGs. In BG2 for example you dont have enemies wake up their sleeping allies or throwing healing potions at them. They also tend to ignore your backline and it's pretty easy to tank them. Also bigger fights with many enemies and big parties will become a slogfest especially if Larian will add proper reactions. There is a reason why Solasta went with a 4-person party too, imagine how many pop-ups for Bend Luck you will get in a turn in a fight that has more than 12 people?
