Pans are measured according to the diameter of the lip, not the diameter of the cooking surface. Most home burners can only comfortably fit a pan of around 12 inches in diameter. Because of its straight sides, a 12-inch sauté pan will also have a large, 12-inch-wide cooking surface (about 113 square inches). A skillet, on the other hand, loses at least an inch on each side, making the effective cooking area only 10 inches wide (about 79 square inches). This means that, given a skillet and a sauté pan of equal diameter, the skillet will have 30% less cooking area than the sauté pan. That's not an insignificant amount.