Down bags have a fill rating that indicates the amount of volume one ounce of down occupies. The higher the fill-rating, the loftier the down. It provides more warmth with less weight.
Mummy-shaped bags taper towards your feet to reduce volume and hold warmth closer to your body. These bags can feel restrictive to some. A semi-rectangular bag can offer more room to move.
Down loses its insulative properties when wet. Some bags feature hydrophobic treatments on the outer fabric or on the down itself (like DriDown) to keep the down insulation dry.