You can see why the stock hat is so heavy; it also contains the parking brake drum. The new part has an alloy hat, and the parking brake shoes ride on the hard anodized aluminum. Although this works fine for use as an emergency brake or a parking brake, you probably don’t want to go around doing e-brake initiations all the time.
The hard anodized inner drum surface is very hard and long wearing, but it is also thin. Once you wear through it, the soft aluminum will get gouged easily by the brake shoes. In normal use, that will never happen, and we will take the lighter weight.
An important feature about the pads is that they are much taller than stock and stick out below the caliper using all of the rotors friction ring surfaces. Thus they have much more braking area or pad surface volume than stock and fully exploit the larger front rotor and the wider friction face of the new rear rotor.
The thin black things are Teflon coated, dual layer stainless steel shims that go behind the brake pads that act to reduce rattle and squeaking.