Dr. Charles' entry on the genetic link to "elves" got me wondering about other mythical creatures.
Some 11-18 thousand years ago, a 3'6" tall tool-wielding "hobbit" roamed the island chain of Flores. Carl Zimmer of the Loom has a little timeline of what happened since their discovery and controversial claim to fame as a new species of humans. There is a big debate over whether or not the Homo floresiensis are pygmies, like those from Africa, or small children with microcephaly.
Shifting back to stories of fantasy, J.R.R. Tolkein's dwarves live in mountains where their short stature helps them survive the same way pygmies in the forest or on islands do. Dwarves have a fondness for ale, axes, beards, and gold.
People with dwarfism might not have the same predilictions, but the mythic dwarves are likely derived from the phenotypic characteristics of achondroplasia (Medline, OMIM entry). The autosomal dominant trait involves a genetic mutation of FGFR3, limiting bone formation from cartilage, particularly in the long limbs of the arms and legs. This results in short stature, disproportionately short limbs, a large skull and prominent forehead.
While little people have a problem forming bone, people with acromegaly (Mayo Clinic, OMIM) have a problem forming too much. Gigantism is caused by excessive secretion of growth hormone from the pituitary gland.
I remember reading a comic strip from Calvin and Hobbes referencing this... but people with gigantism don't grow to be skyscraper sized.
Andre the Giant is a famous example. He was a professional wrestler and he was in the Princess Bride. He was a very large man at 7' 2". This did not go without any consequences... his immense weight took a toll on his joints. His body grew too big for his heart and he died of congestive heart failure.
I'd like to add more, but I'm plumb out of ideas.
Stay tuned for the medical explanations of the occult, just in time for Halloween!