Carboys come in many sizes, but all of them have generally the same shape and features, including a small neck. The small neck of the carboy may make them difficult to clean but it has one huge benefit. Small surface area. Six gallons of wine in a more traditional vessel, like a bucket will have a surface area of over 100 square inches, which is exposed any gas that happens to be in your carboy. Another six gallons of wine topped up to the neck of the carboy will have a surface area of less than 1.5 inches. This small surface area dramatically slows the rate of oxygen absorption into the wine.