If the robot has more mass, then it will take a bigger force to lift up another object. If the robot is going to lift something up, it will use its arms, and its arms have mass, and the object has a mass as well. If the robot's arms have a mass of 10kg, and the object has a mass of 12kg, it will use a certain amount of force to lift up the object using it's arms. If the the robots arms were 5kg instead, it would take less force to lift of the same object using its arms.
