They are the most thermal efficient bags available. They are made to fit close to the body by tapering down the legs. The taper cut allows for less circulation of air.
Low circulation creates dead air which retains heat best.
There is also a hood that covers the head, to create a cocoon around the body trapping the valuable heat. Rectangular Bags are best suited for warmer weather camping. These bags are cut for comfort and not efficiency. Rectangular bags usually do not have hoods, which makes it less desirable in cold weather. Tremendous amounts of heat can be lost through the head.