You don't level up faster when solo. XP is not divided, everyone gets the same amount.
That is stupid.
There s no point in playing with less than a full party.
Unless you want a challenge.
I think that is the point.
I just wish Larian limited some item equips and character abilities to specific classes. I kinda dislike that every single class can pick locks and effectively pickpocket.
If anything, it is stupid that the game can be played with a single character efficiently.