It sounds like you are saying a gallon weighs 128 oz instead of contains 128 fl oz. On a larger scale (like gallons) you can't interchange weight and volume measurements with water. 8 fl oz of water weighs about 8.35 oz. Multiple that by the 16 cup to a gallon and you end up with a weight of...
Your co2 tank is filled at high pressure. The cool thing about a gas is that it can be compressed. You will be filling your keg at a much lower pressure. It won't empty your tank.