The original Endoskeleton Premium Format was made of metal and solid polystone. Of course it's going to be a lot heavier than this.
Most of Sideshow's current pieces are far lighter than those of old. It's a cost saving technique, as well as reducing the cost of shipping due to the increase in shipping costs these days.
Nevertheless, those parts I listed are definitely polystone and not plastic, but yes he is still light.
Lightweight doesn't automatically mean cheap, and polystone is dirt cheap material. They don't get a discount on shipping due to weight, it goes by size. No matter the actual weight, they get charged for at least 20-25 pounds just due to dimensional weight.