I'm not entirely sure that the original Heavy Iron Sword was necessarily superior to its progeny, the Heaven Sword and the Dragon Sabre (especially the latter, which had comparable weight and mass). When Gwok Jing and Wong Yung forged the Heaven Sword and the Dragon Sabre, they did not rely entirely on the raw material provided by the Heavy Iron Sword. That would not have been enough to forge TWO new weapons of the combined weight and mass of the Heaven Sword and Dragon Sabre. Additional metals were incorporated into the material that composed the Heavy Iron Sword, creating an alloy likely superior to that of the Iron Sword's.