Here's why:

* Inertia is the tendency of an object to resist changes in its motion. This means an object at rest wants to stay at rest, and an object in motion wants to stay in motion at the same speed and direction.

* Mass is a fundamental property of matter that measures its resistance to acceleration. The more massive an object is, the harder it is to get it moving or to stop it once it's moving.

Therefore, the greater the mass of an object, the greater its inertia.