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.
Yes, and there are other things you can do too to increase the difficulty of a game. But you don't HAVE to add more ennemies.
Some battles in BG3 already have more ennemies than hack & slash games. It would be terrible to add even more goblins
if there are enough enemies, then it would mean placement & positioning, add variety to the encounter and stats changes. I'm not sure at the moment if Larian did any stats change to the bestiary. If they did, then i see no issue here if that's the case.