Well i always thought that this game would have the same number as the BG1 and BG2 that was actually kinda unique for the time if im not Wrong.. after that people start to make this rule of 4 man party and yu start to see this in all games.. But i do understand why they did this way, the reality is that they want to extent the Gameplay time making yu trying out a Bunch of party compositions in differents gameplays like i did. I already beated the game 7 times by now. more then 500 hrs played.. already see a lot and geting bored. But 1 thing that already is in the Game that i think would not Brake the Game is increase the Party for at least 5 man Becouse this already Happens in Many times in the Game will not Say becouse spoilers.. but yu can at least 4 or 5 times during the game get a 5 man party for some especific quests.
Man I remember Dungeon Siege 1, with 7 chars and a mule to carry all the loot. It was chaos XD I loved it, but nowadays I too prefer less characters, 4 or 6 is perfect. It is easier to focus on your characters. With more, same happens to me what happens to my long Civilization playthroughs, I get so many cities, I stop caring about developing them the right way, and just do random stuff. Same with leveling up 8 characters, at one point I can't be bothered and it becomes a chore.