Piston balancing

Why is it when balancing as a wristpin and piston as a set that you wouldn't remove the material from the wristpin instead of the piston?

I generally remove material from the piston as it's just easier to remove from here. The downside of course is that you'll need to remove more material to reduce the mass compared to a steel wrist pin. If you choose to remove material from the wrist pin you need to do this from the ID of the pin and make sure you leave a smooth finish with no potential stress raisers.