Board Thread:Versus Debates/@comment-31171723-20170128170340/@comment-25122992-20170211214401

Some pages aren't shown in the right order, but anyway, nothing really indicates the sword was expanded in the moment it pierced Oro. It was already long enough when Susanoo pulled the blade out of the gourd in the beginning of the fight against the serpent. In fact, a distance between the snake's head and Susanoo hand holding the blade rather indicates it was just thrusted into the target.

P.S. Also, we can see Oro making some sound when the blade runs through him which means he reacted to it indeed. It seems like a sound of surprise, so Oro really didn't expect from Itachi to use such a "weak" attack.