New Kun & Ulic wank
During the height of the Great Sith War, Thule served as a base of operations for the Sith Warriors, and much of the planet's crust became imbued with the residue of the dark side of the Force.
-The Complete Star Wars Encyclopedia
Thule is stated to be more than twice the size of Earth(12,742 km):
DIAMETER:
27,539 km
- Geonosis and the Outer-Rim Worlds
The thickness of the Earth's crust is 30 kilometers, which if we make a loose comparison would be roughly 70 for Thule.
All-in-all, it's a pretty immense corruption feat, especially given that they couldn't have spent more than a few months here given the Sith War lasted 5 years.