Damage to champions

Real simple question, I know it's been asked probably a thousand times. Anyway specifically I'm learning jungle and have been practicing kha'zix, rengar, assassin type junglers. I don't find I do a lot of damage to champions. Should assassin junglers be high on the damage to champions list? For example just played a kha game and went 15/5/13, was a 55 min game and did 23k damage to champs. That seems pretty pathetic. I'm pretty sure I'm just over cautious and play really safe.

What are your guys thoughts?