In these days of weight-saving, people often get away with the smallest reserve that they can, as they hope that they will never use it anyway, and they have to carry it around in their kit. If they do use it, however, the descent rate they get can be scarily fast, and schools often advise buying a bigger reserve than that recommended for your weight.
ITV have now come up with an EN-certified reserve, the ‘Light Speed’, that is lightweight, but almost twice the surface area of other brands for the same recommended loading, and with a correspondingly lower descent rate.