We also assume that a larger, heavier bag is the warmer bag. So many factors contribute to the warmth of a bag - type of fill, amount of fill, the loft of the fill, the bag's shape, design, and construction. In fact, sleeping bag product engineers and designers strive to develop bags that have less bulk. It is because of the superior quality and design that a lighter, less bulky bag can be as warm as a bag that's over-stuffed with less expensive fill. Selecting the right sleeping bag depends on how, when, and where the bag is going to be used, and type of person who'll be using it.
Size is always an important consideration.
As If You're Buying Clothes. Sleeping bags are most often classified by their shape. Most quality mummy bags feature hood and neck closure with a drawstring that pulls the hood snug to your head. Also look for a self-repairing coil zipper and a zipper baffle which is an insulated flap that covers the zipper on the inside to help keep the warm air in, cold air out. And if you're buying bags for more than one person, consider styles that let you zip two mummy bags together with left and right-hand zippers.