From the Manufacturer

The Ultra Rebound Speed Bag is based on the very same principle as Galileo-Feets pendulum device. An additional layer of leather at the base of the speed bag provides the increased weight distribution for a faster, truer rebound off the platform and will challenge even the most seasoned speed bag practitioners. Available in Extra Small (5-Inch x 8-Inch), Small (6-Inch x 9-Inch), Medium (7-Inch x 10-Inch).