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.