If you look at your entire arm and hand, it would be reasonable to think that more cortex is dedicated to sensory and motor functions of your arm (because it's larger) than your hand. Actually though, our sensory and motor cortices are mapped based on the density of receptors in a given body part. 
 

The sensory homunculus - shown in the picture - maps out a strip of cortex called the primary somatosensory cortex, or postcentral gyrus. The motor homunculus maps out the primary motor cortex, or precentral gyrus. We have one sensory and one motor strip for each hemisphere. The left hemisphere is associated with right body functions and vice-a-versa.
 

If you noticed, this little guy has monstrous hands. Our primary motor and somatosensory cortices have a large space dedicated to hands in order to help serve all of functions that they do. Just think about how easy it is to pick up a toothpick with your fingers compared to your toes.

 

Another strongly represented body part in our cortical homunculus is the tongue. We have tons of sensory receptors dedicated to taste, and need many motor receptors for fine movements during eating and talking.