Contrary to common belief, the Sahara desert is not the largest desert on Earth. The most widely accepted definition of a desert is a place with less than 10 inches / 25 cm of annual precipitation. This holds true, among other places, for Antarctica, which gets even less precipitation than the Sahara, and is certainly larger. The fact that there's plenty of (frozen) water around because the precipitation of thousands of years has piled up is immaterial.