Why i cant do more damage ?

So BD isn't my main but i really like to play it She's at level 13 with talents ( left right left ) And my highest damage with it is 180M My cultist on the other hand is at level 13 same as BD but shes not comparable to him at level 13 my cultist's highest damage is at 900M~1B... So my question here is why my BD's damage is much lower than that of the cultists when they are at the same level ? and i know i played her right this game