As a ball rolls downhill, its potential energy from its height is converted into kinetic energy of its motion. Potential energy is given by mass x gravity x height. So for the same height, a ball with higher mass will have more energy. A bowling ball is heavier than a soccer ball and will have more energy.
They are traveling at the same speed, but the bowling ball has more mass. The bowling ball has more kinetic energy because more force is needed to stop the rolling bowling ball. Two balls with different masses moving at the same speed have different amounts of kinetic energy.
