I imagine, the problem comes in from there being wildly different sizes of both, dogs and chocolate.

I once heard a story, where a small dog had eaten chocolate, so the owner called the vet. And at first, the vet told them that it's probably fine, but wanted to know how much chocolate the dog ate.

And when they then told the vet, that the dog had eaten a full-size chocolate bar, suddenly it was treated like an emergency. As in, they had to bring the dog to the vet immediately, got treated before everyone else still waiting, and the dog was given a drug to make it puke out all the chocolate.