The tabletop recommends a party size of 4, and in the turn based system, trash encounters would become a slog if they were balanced for 6 players, since they would have to add more mobs in each encounter to compensate.
"The preceding guidelines assume that you have a party consisting of three to five adventureres... If the party contains six or more characters, use the next lowest multiplier on the table." - Page 83 D&D Dungeons Master's Guide
5e considers 4-5 to be standard while 6 or more being optional, totally up to the players and DM for party size. So Larian at the very least can give us max 5 party members if they're being picky with the rules. The reason why many players want the option (keyword option), is because it was an option in the original BG 1 and 2. This is a Baldur's Gate game, doesn't matter who's developing it. I also would like to mention that many official WofC D&D adventure modules of both current and past editions are geared for 4-5 and 4-6 players.
I don't know how much more literal I can get with actual sources. I don't understand the issue of not favoring max 6 party members as an option.
I understand it's going to take "resources" to balance and tweak the game but Larian has both the time and resources.