The large ball collides with the small ball after rebounding from the floor, transferring its momentum and kinetic energy to the small ball. The small ball is then able to reach a greater height than from which it started. The maximum gain in velocity for the small ball from the collision is three times what it has upon reaching the floor. Thus, the maximum height to which it can return is nine times the height form which it was dropped. The closer the small ball's mass is to the larger ball's mass, the less this return height will be.

Jearl Walker; The Flying Circus of Physics.

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