With respect, it doesn't work like that, and there are standards to cover differences in weight too. All part of NICE.
With all respect to you man, it DOES work just like that. And it illustrates how your Vet might be a better physician than your family doctor. All medicine dosages, if they were properly given, would be calculated on a per lb body weight basis.
As I've said, human beings are not standardized, but many physicians only give weight variance minor consideration.
Having taken both physiology and anatomy, I'd feel more comfortable with many Vets (as opposed to the average doctor).
And having grown up with a significant part of my family employed in the medical profession, I worry when I see too many people blindly following their doctors suggestions.