Here's why:

* Mantle: This layer is the thickest layer, extending from the Earth's crust to the outer core. It's made up of mostly silicate minerals and makes up around 84% of Earth's total volume and 67% of its mass.

* Crust: The outermost layer, relatively thin compared to the mantle.

* Core: Divided into the outer core (liquid iron and nickel) and inner core (solid iron and nickel). While dense, the core is smaller in volume than the mantle.

Therefore, the mantle, due to its sheer size and composition, holds the majority of Earth's mass.