Here's why:

* Mantle: The mantle is a thick layer of mostly solid rock that lies beneath the Earth's crust and above the core. It makes up about 84% of Earth's total volume.

* Crust: The crust is the outermost layer of Earth and is relatively thin compared to the mantle.

* Core: The core, while dense, is a much smaller sphere than the mantle, making up about 16% of Earth's volume.

So, even though the core is incredibly dense, the sheer size of the mantle gives it the largest volume.