I'm saying other people don't get a multiplication effect. When partied up the xp probably gets shared, and at most there's a bit of a bonus based on the number of participants, but the MC is getting the whole amount transferred back to him, and the girl is getting a cut of this larger pool. We're going by the numbers she gave, but she might not be all that reliable, it's possible that she's getting like a 20% cut of everything being killed by the slimes and the douchebags that tried to rape her, and it just seems to her like her faulty math is right.
As for why his monsters can get stronger without keeping any of their xp, it's because he is getting stronger, and it's the added strength of his version of Domination amplifying their power. He's like a demon lord, the stronger he gets, the stronger the boosting effect he gives his subordinates.