As Ging demonstrated with the changing shape of his nen on his hands with great speed, he has mastered manipulating his nen to an insane degree. And Leorio's ability is just like that so copying it was a piece of cake. He could easily copy any ability that's just pure nen in form like let's say Gons Janken or Uvo's big bang. These techniques are basically just manipulating the shape of your nen.
Transformation on the other hand is waayy different. It already takes into account the personality of the user affinity etc etc and it's not something Ging can copy unless of course if he's a specialist like Kuroro.