I think it probably does relate to safety. Big dogs are more off putting to burglars than men or women are. A dog is also more likely to hear an intruder than a person. When asleep your brain is going to find it easier to differentiate dog noises from a human intruder. Where as if there is a human moving around it's going to be more difficult to detect whether it is a danger an something you need to wake up for, or just another human member of the household. So peoples sleep may be disturbed more by other people than dogs.