This is something I've struggled with and struggled with in comparing human medicine to veterinary medicine. In veterinary medicine, all animals are dosed based on the mass (kg "weight") of the animal. Scaling is used to in veterinary medicine to provide doses based on mass even in very large animals.
In human medicine it seems much more like its an adult/child dosing with many different medications (antihistamines, antibiotics, certian blood pressure medications, etc). Just as an example: I'm a 300+ lb man. If I get an ear infection and I get the same cipro prescription my 132 lb wife gets. It doesn't make much sense.