They're not really that much lighter; they could get pretty thick. They are lighter than true 2-handed swords, but are in about the same weight range as European swords of the era. You'd be surprised how light realistic swords are. They were made to be carried 99.9% of the time and used .1% of the time, and are defensive as much as offensive weapons. Speed and controllability was very important. European swords had 1000s of years of refinement behind them, so they tended to resolve flaws with each design. It eventually led to the rapier, which I would choose over a katana in an unarmored duel any day.