It's way more complicated than that: there are differences in metabolism as well as size. It varies from drug to drug too, but as a rule of thumb dog needs about twice the human dose (in mg/kg). A 60 lb dog therefore needs about the same as a smallish adult.
Large dogs can be the same weight as small humans, so I'm not sure what accounts for 1/10th. Even medium-sized dogs like labs are about half the weight of a healthy, average-sized person.
Depends on the drug, but their metabolism isnt identical to ours, and humans can often require significantly higher doses compared to other animals when controlling for dosage per weight unit. But again it depends and there is no hard and fast rule as far as I’m aware.
What I’m getting at is that for most animals the owner can be compensated for accidental death easily. (How much is a head of cattle at auction?)
Not that this explains all the discrepancy, but it may explain a non trivial percentage.