The trains are multiple units, so each car has its own powered motors. (Other than trailer cars, but it's pretty trivial to just add more motors.) The logic is

The trains are multiple units, so each car has its own powered motors. (Other than trailer cars, but it's pretty trivial to just add more motors.) The logic is pretty simple. Less train cars means less people per train, which means you can have shorter/narrower platforms, less escalators, fewer exits, etc. and significantly reduce the structure size.