Recent trend in RPG design is around an assumption 2 to 4 players with 5 or 6 being large. This includes D&D as all the official published campaigns have been balanced around a 4 person party (if you have 5 or 6, the published challenges are significantly easier than they are planned to be.)
My suspicion is that this is due to the average age of roleplayers has been getting older, with a lot of players in their late 20s and early 30s, and many players getting up in their 50s, 60s, and even 70s. My theory is that this makes scheduling larger groups difficult and the effective length of sessions tend to be shorter, to account for next day work or fatigue from earlier, and thus have to be more efficient, which, again is easier to do with smaller numbers.
This player range has also been common with a lot of console games due to practical tech limits, especially console games. I think BG3 is a PC exclusive release so that might not be a huge factor, but it plays to the experience of a lot of video game designers. A lot of single player games with multiplayer co-op options tend toward 2 to 4 players similarly.
I expect BG3 can be stretched to a 6 player base, but that will mean larger enemy counts, larger in general battles, longer battles and other such things. I don't mind any of this, as I'm content with long battles and quite enjoy them. But several people on these forums have already complained about the fights being as long as they are.
I am up for 6 person party, because I like the companions I've seen and would like to keep up with more of the existing NPCs than we currently will be able to, but I doubt we'll see it. That feels like the sort of thing that's outside the scope of probable changes we'll see at these stages.
