The weight of a down sleeping bag, in addition to the amount of filling, is the weight of the sleeping bag fabric. To a large extent in order to have a ultra lightweight sleeping bag, you need to reduce the weight of fabric
Outer fabric material: The outer fabric generally needs certain strength to ensure that it can be used for a longer period of time. Due to the requirements of outdoor condition, most of the time it will be treated with water repellent treatment, that is, it will not be quickly wetted with a little water. This waterproof is not enough to against rain, so if you need to camp without tents, the purchase of GTX material sleeping bag sets is better.
Lining material: usually more than 200T is appropriate, because if the index of the inner lining material is lower than 200T, individual down will be worn out, however, thick fabrics still will be drilled at the seams. Now a light-weight sleeping bag with a zipper covering the entire weight of about 400 - 500 grams, the extreme can down to 300 grams.