Small dogs have a wider range of hearing than large dogs.
Dogs' ears, in addition to being able to move in a variety of ways, can hear considerably higher frequency noises than human ears. In fact, dogs can hear noises that are twice as far away as we can—and it looks that tiny dogs are even better at it. This is due to the fact that the smaller a mammal's head is, the more frequencies it can take up and compare in each ear. That is how they (and us) determine the source of noises.