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.

Here's some FDA guidance on how to translate doses from animals to humans: https://www.fda.gov/media/72309/download

Even more than twice. My dogs, despite weighting 5th of me had similar to mine dosages of antibiotics and antihistamines

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.