Here's why:

* Composition: The inner core is primarily composed of iron (Fe) with a small amount of nickel (Ni). These are very dense elements.

* Pressure: The immense pressure from the weight of all the layers above it compresses the inner core even further, increasing its density.

* Density: The inner core has a density of around 13 g/cm³, which is about 5 times denser than the Earth's surface.

While the mantle is the largest layer by volume, it's less dense than the inner core due to its composition (mostly silicate minerals) and the lower pressure at its depth.