No, you cannot. Your dog would NOT be getting the correct amount of the medication. It is NOT as simple as cutting the medication in half or at all. The truth is, he needs the CORRECT dose for his weight! Don't start messing around with his medications. That can be dangerous and it is NOT recommended.
And the same is true for Frontline... you cannot give a small dog 3/4 of a larger dog formulated Frontline. It has NOTHING to do with the actual amount of liquid, it has to do with the concentration! ALL Frontline vials regardless of the weight of the animal contain the same amount of liquid, but VERY different concentrations of the medication.