There are two contenders for the world's smallest mammal, depending on whether one is going by length or weight. The world's lightest mammal is the Etruscan shrew, while the shortest is Kitti's hog-nosed bat.
The Etruscan shrew weighs 2 grams or less. However, like all shrews, it has an extremely rapid metabolism and eats twice its weight every single day. Although it is the lightest mammal in terms of average weight, the Etruscan shrew is 2 inches long. According to Sci Tech Daily, the Kitti's hog-nosed bat, or the bumblebee bat, is only 1 inch long, making it about the same length as the bumblebee it is named for; the average weight of a Kitti's hog-nosed bat is also about 2 grams.