Xiphos Sword Why Did Swords Become So Long During The Middle Ages?

Why did swords become so long during the middle ages? - xiphos sword

A stab in the belly of an old short sword, it would be better to kill a specific focus on the arm of a medieval sword, when he cut his arm still alive!
More short swords like the Roman gladius, or particularly swords ancient Greek or Kopis Xiphos could still have a very good tip.
So, why the Middle Ages, swords sheets of 4 feet or more in legenth?
I think it unnessceary and inneffective, or am I mistaken?