Typically more than 10% less total insulated area (due to intricate tapering and the use of a false bottom), which also means more warmth from a given fill amount. A highly customizable fit allows you to remove extra weight and get a trim fit. A fit that is cut for your body reduces the empty space inside the system, which is good because all that empty space is inefficient. It is difficult for your body to heat up all that extra space.
This also allows you to size a system large enough for different sleeping positions, such as fetal, stomach, and one knee up. While the top and footbox are kept trim for efficiency, the torso area can be sized roomy to allow for your knees to bend. If you follow the sizing instructions on the “sizing sleep systems” page, you can size it large enough for you to bring both knees up to full fetal position inside. This is an extremely efficient position for retaining warmth, if it is comfortable for you. This sizing also allows for the popular face down with one knee out to the side and the other leg outstretched.