When it comes to describing the action of "over filling," the opposite could be "under filling," "leaving empty," or "filling partially." Under filling refers to not filling enough, leaving an insufficient amount. Leaving empty would be not filling the container at all. Filling partially simply means filling the container halfway or partially. It's important to pay attention to the filling level to ensure that the desired outcome is achieved. Overfilling can cause spills, waste materials, danger, and unnecessary costs, so it's essential to differentiate and use the opposite of overfilling when appropriate.