I don't remember how it work in DoS and I don't know how it works in D&D but in the older BG games, the experience you acquire is divided between the member of the party. i.e if you kill a wolf that rewards a solo player with 600 xp, every member of a party of 4 would have 150xp and every member of a party of 6 would only have 100xp.
At the end of the game, a solo player could reach a higher level than a party of 6 (if you consider another/no level cap). You can finish BG1 and 2 with only 1 companion. There are no "cheat" skill like lone-wolf or things like that if I'm not wrong and multiclassing allow a limited team to face many different situations.
If we project this exemple in DoS or in D&D5e : does every (alive) characters after killing the wolf will acquire the 600xp or is it divided the same way ?
That said, I don't really understand why it should be impossible to balance the game whatever the number of companions you choose to play with.
Do you have clues about that point ? Maybe it's related to the rules or something else I don't think about ? To have a concrete exemple, isn't it possible to play the exact same D&D campaign with 4 or 6 characters if you have 50% more XP while playing with only 4 ?