They might have gone a bit overboard with their estimations, but the study is from the National Institute of Infectious Diseases in Tokyo, Japan. NIH have just archived it.
The amount of theobromine we can metabolise is around 1000 mg/kg vs. 300 mg/kg for dogs, which leads to a much slower build-up and therefore it takes quite a lot more for it to reach toxic levels. It has less to do with size and more with how the body deals with the TBR, rabbits and male mice...