Because its probably a fake rubber or plastic stunt sword and does not have the heft and weight of a Japanese samurai sword.
Still asian made swords whether Chinese, Japanese or any other weigh less than there their european Knight counterparts carried.
Sulu being Japanese and having a samurai sword is consistent with his ethnicity where in the old series episode being drunk he started thinking he was the leader of the three musketeers and had a french style fencing sword. Which is just plain silly. In real life you can be trained in body and mind in arcane martial disciplines and weapons training, so why not a samurai?
I mean this series is pure fantasy anyway just like star wars. Even though set our primary universe the science of trek is laughably implausible if not outright impossible. Trek is just as much space western, space opera/ fantasy as star wars. Just not sword and sorcery, and bad episodes with black cats and black magic crossed that line.
I mean how is Sulu swinging around a samurai sword any more silly than kirks karate chop and fake kicks and punches, or Worf's swinging around a batlith or however it is spelled.
Sure it is a lot more credible when a gymnast trained in kendo like Ray Park who played darth Maul in the phantom menace is swinging around a sword when he clearly knows how to do so. Unlike actors getting a crash course like Mark Hamill on the old trilogy being trained by Bob Anderson, and on the new trilogy people like Hayden Christensen trained by Nick Guillard. Park was great as maul in star wars,toad in x-men. and as snake eyes in gi joe will being awesome.
As for John Cho from Harold and Kumar who is entirely miscast for this role being a swordsmen is laughable.
I mean he got slammed for not even being Japanese in the beginning, but i just only picture him as that dumbass from those 2 stoner movies. I mean it is not Harold and Kumar help spock excape from Guantanamo bay,lol. Or Sulu and Kirk go to white castle.
Then again Simon Pegg as Scotty though his scottish accent is real is awful on all levels. Quinto aka sylar as spock is a joke. a sexed up uhura played by saldana does not give respect to the role nichelle created. Urban is almost believable as a charicature of deforrest kelley, and pine nowhere excudes the coolness of Shatner as kirk.