Anyhow. Yes! Of course it would be better if the game was built around your preferential party size, i am with you on that. But that is going to take much more time and manpower rather than just implementing a warning at the beginning of the game stating that it will probably become easier and a little less balanced if you choose a 6-party setup. Since 6 character party is already possible in the "tutorial", it just seems arbitrary and cruel against us that want to experience as many companions as possible to lock the party to 4.
Here's where we disagree. I highly doubt that it'd take "much more time and manpower" to implement scaling exp. It's likely only a few lines of code, especially since the game already tracks individual character exp. - Exp granted per character = old exp * (4/party size). Actually, splitting exp should be implemented even if Larian doesn't allow 5- or 6-person parties, so that players can play with 3 or fewer characters and gain exp faster.
If Larian made no changes to encounters or exp, sure I guess it strictly improves the game to include a setting: "allow 6-person parties." But that is the bare minimum, and Larian could easily do better. Personally, I probably wouldn't use the setting as it'd remove a lot of the challenge from the game.