Oil cooling
Removing heat from the interior of an electrical machine is difficult with air, and the dimensions of the stator and rotor are governed as much by the need to keep eddy current density in check to keep heat production sufficiently low.
This is why Teslas can’t keep doing their party piece acceleration all the time; the motor gets too hot and it takes time for the heat to bleed away.
I can see oil cooling being a lot better at heat removal, allowing the stator and rotor to be smaller and thence lighter. I wonder if they flood the genny with oil; quite impressive for something spinning that fast!