Here's why:

* Thinness: The Earth's crust is actually quite thin compared to the other layers. It's only about 3-50 km thick.

* Density: While the crust is solid, it's composed of relatively light materials like granite and basalt. It's less dense than the mantle, which is the layer beneath it.

In summary: The Earth's crust is thin and relatively light, not thick and heavy.