A lot of the replies here are based on shorter/longer combat, convivence to the player, or being more aligned or not with table top gaming.
I am under the impression that the developers created a 4 man limitation to force choice on the players and make us have to decide what we are willing to lose.
Otherwise if the party size expands, we have TOO many tools available to us. If I have a character to excel at every skill available, I can just swap in and out to ensure highest chance of success at every social or combat encounter.
Choices like this are to encourage weakness and force limitations on players so that a challenge is presented and deliberate choices are made. It feels bad to have to give things up, but it is also good game design.
If a DM/GM gives everything to the players, there is no challenge and no sense of struggle to overcome which in turn becomes a sense of accomplishment.
Basically, if you let players be gods and perfect, the game is boring after a while. Limits create opportunity for variations and new experiences as you try new things for future playthroughs. Otherwise, why provide the illusion of choice if you let the players get everything, just get rid of dialog options and skill checks and make the game linear (I am sure you all don't want that).
I think party size was determined based off the old initiative system. If your whole team went at once and you had 6 players, you could destroy the opposition before they ever attacked. Now that each combatant rolls initiative, a part size of 6 should be fine. Your other points are refuted by BG1 & 2. Those were great games.
I made my comments from assessing the previous title, divinity original 2. Same party intro, make friends and then split the party with your select few occurred there.
Additionally I don't believe the existence of previous games "refute" the points. It is a game development decision. Give players everything, or give players nothing.
Both systems have been used in many games. I know JRPG games are very fond of grinding it out and getting every level, every class, every character, every item. Those games are largely popular in both western and eastern gaming communities.
I didn't sufficiently advocate for the limited acquisition model, by putting too much opinion on it and I too put that above one or the other. What I tried to do was bring insight on to "why" a decision was possibly made to limit size, beyond just initiative system or other limitations.
It is fine to disagree that a small party or a large party is better. It becomes a point of intent. Do they want to limit resources, or do they want to expand them? To us the players, are we fine with limited resources or do we think expansion of resources is better? Balancing and addressing those concerns is good stewardship of a developer.