If she used the voo-doo spell on a guy's penis and forced him to be her enforcer, that would be more interesting.
Misbehave? Shrink penis. Angry at a rival bitch? Have him fuck her and enlarge penis mid-fuck. Horny? Fuck while constantly shrinking and enlarging penis inside herself. (Then accidentally maximum penis...)
A good concept for shrinking. But the short comic means not much is done with it.
She could have shrank him more. Forcing him to serve her to get his size back. Or made him pay extra and every time he refuses she shrinks him and increasing the cost to get big again.