A 10-20 pound weapon was not that unusual during this period, as the larger swords and pole weapons weighed in at about the same range. By the 17th century, your average infantry musket weighed 10-15 pounds. Modern infantry firearms, for example, have tended to range from 8 to 20 pounds, although even more weight than that is usually carried in the form of ammunition. Despite this, don't get the idea (easily acquired from movies) that the medieval knight was overloaded and virtually immobile on foot. In fact, a complete suit of armor, plus weapons, weighed no more than about 80-90 pounds, and was usually closer to 70 pounds. Modern infantrymen have frequently gone into action with that much, and often more. And the infantryman's load is disproportionately between his shoulders, while the knight's kit was rather well distributed about his frame.