It's funny that you think that they are building their encounters even remotely based on D&D tabletop rules.
First of all, I am not thinking that. I explained why you are wrong to assume that in actual DnD campaign a smaller party will outlevel a larger one. Because in actual DnD campaign encounters are tailored according to the party size and level so that every player will earn a specific amount of exp for each adventuring day. You split the exp, but the total amount of exp a larger party earns is supposed to be much higher. In the video games where the encounters are mostly static and do not adapt to the size of the party having a party of 6 and 4 will require creating different sets of encounters for each party size to achieve that. Using raw xp split without it will cause unbalanced and frustrating gameplay. That's my point.