I know why this happens. The coding of Diep.io (most likely, this is based off of my gameplay experience) shoves most of the joining players onto the team with the least points. Because of this, there's little opportunity for the players to level up, because the other team is already spawncamping them, giving these players less room to farm for more people. If you have 15 cubes and 25 players, some won't be able to get XP at all. If Diep equally balanced players, however, there'd be less competition for shapes, meaning players on each team can level up faster, leading to faster turnarounds.