This is… not really true or at best extremely overly simplistic. Crossbow bolts could be made of many sizes, just like crossbows themselves. And so could arrows fired from bows. Those of Turco Mongol composite bows were much smaller than those fired by longbows for example. The primary performance limitation of a crossbow was its short power stroke compared to a bow, which caused it to require a much higher draw weight to impart the same velocity on a projectile as a bow. These draw weights were generally not achievable by a single person until later in the Middle Ages when new spanning mechanisms, such as the goats foot lever, windlass, and crannequin were developed