Larian doesn't even need to change the encounters for a different number of companions. Simply divide exp evenly between all party members that participate in a fight. Maybe put a disclaimer somewhere that "BG3 is intended for and encounters were balanced assuming 4 party members. Parties of 5 or 6 will gain less experience per member and parties of 1-3 will gain more, but your difficulty may not exactly match what we have tuned the game for."
If they wanted to get a bit fancier, then they could try to find a better equation for dividing exp: e.g., divide the experience by (party size)^2 instead of just party size. They'd probably want each member of a party of 6 to get about half the exp that a party of 4 gets..? There exists some equation that effectively balances most combats for any party size.
If they wanted to get even fancier, then they could take inspiration from DAO and auto-level companions you don't take with you to [your PC's level] - 1. This would open up some shennanigans where you solo combats to get a lot of exp, which then auto levels your companions. But eh, I'm fine with that type of exploit.
Part of the issue, is that the entirety of combat is centered around Larian's homebrew rules moreso than 5E, so adding a 5th party member isn't just about adding another person who can attack/cast spells etc. It's another person who can push, create surfaces, carry/throw barrels, abuse broken sneak.
That is going to completely take the existing imbalance, and ratchet it up more than a few notches.