If you are going to use a pastry bag or paper cone to decorate cakes, it is essential that you are especially careful about the

I believe the answer is: consistency of the icing and the pressure you exert on the bag or cone

Icing with low consistency tend to be more liquid and would come out more with a little force and vice versa.. The pressure that you exert on the bag or cone would determine the force that could influence how much icing would come out in one press