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 measure of the amount of matter in an object. The more mass an object has, the harder it is to accelerate or decelerate.

Therefore, the more massive an object is, the greater its inertia. A heavier object will resist changes in its motion more strongly than a lighter object.