This is exactly the claim that @GM4Him is refuting, by pointing out that there is no such thing as "standard party size" for a D&D game module because the exact same module can be played tabletop with party sizes ranging from as small as 4 to as large as 8.
Well, I did not look at the whole issue as a "standard party size in D&D". I'm actually pretty far from D&D (never even played BG1/2) and my knowledge is more about bits and pieces of lore rather than tabletop mechanics.
I don't see a module that can be readily adjusted to a varying number of players. What I see is that BG3 is being built with an exact party limit in mind - four, standard or not. No way around that. Any adjustments to party limit will require more work that just ramping enemy stats or numbers up if were're expecting some balance difficulty-wise. And would I like to see party size changed? Yes. Can I see it happening, even as option? Not really, outside of mods.