The Camelbak line of hydration systems varies in size, including hip bags, satchels, and larger full size bags. The hydration packs carry 80 or 100 ounces of fluid in an insulated pouch.
An alternate use of the Camelbak, is as a small laptop back for the Sony Vaio Picturebook. The Picturebook is the same size as the hydration pack, and the insulation functions as padding for the laptop. Smaller bags, such as the Mule, are ideal for this, for the person who doesn't want to carry a large computer bag.