This is for you physics geeks out there. Given the total mass of a solid sphere and the angular momentum of that sphere, is it correct to say that, if you remove the inner 25% of that sphere, you are reducing the angular momentum of the sphere by 25%?

In the real world, the angular momentum of some small part of the whole object is proportional to its distance from the center, thus the effect isn’t linear - the inner 25% of a sphere have much less than 25% of the total momentum.

Anyway, this usually doesn’t matter in a physics engine, because it tries to emulate only the most interesting physical effects, like reactions to collisions, forces and gravity.

I didn’t test this, but strongly suspect that the physics only takes mass into account when calculating the effects of collisions or applied forces/torque - the resultant velocity and angular velocity are stored in the properties Rigidbody.velocity and Rigidbody.angularVelocity.

No. Because of the angular drag induced by the air which is dependent on the surface and volume of the sphere and not its mass.

Note that in Unity you set the absolute angular drag, assuming you already calculated it from the surface of your object.