[tom-_-]

I imagine reducing the weight of the vehicle would effect it's stopping distance, traction, turning radius, a bunch of other car physics that would in turn produce unreliable data to train on.

I assume much of the training is done in simulation or within a controlled environment but unfortunately the only way to train for city driving is to gather as much real world data as possible and that means "testing in production" with a hopefully alert humans (one for backup) behind the wheel.