The Etruscan shrew weighs only 1.2 to 2.7g so could be considered the world's smallest mammal. Slightly bigger is the Pygmy Marmoset, one of the smallest primates and the smallest true monkey, with a body length - not including tale - of 14 - 16 cm and a birth weight of 15g.
Or how about this tiny Bee Hummingbird who measure 5.7cm and weighs 1.8g
My daughter's Miniature Poodle has just had puppies averaging 200g, which are huge compared to these tiny creatures. Now I have nothing against small dogs, in fact I have always wanted a chihuahua, but I am against breeders trying to miniaturize further, like teacup puppies for example, as they can end up with all sorts of health problems.
So over to you, show us your pets or favourite animals - great or small.