Hi Susan-
In order to understand why a small dog and a giant breed receive the same dosage for vaccines, you must first understand that there are weight based drugs and there are non weight based drugs and then you must also understand therapeutic index which is the comparison of the amount of therapeutic agent that causes the therapeutic effect to the amount that causes toxicity. For rabies and all other vaccines the therapeutic index is 1ml and they are not weight based drugs.
That is why a small breed and a giant breed receive the same dose. Hope this helps!
