Today the story that tells of why humans have different sized bones. So the story goes that the god Quetzalcoatl (the feathered serpent) needed bones in order to create new life. He acquired the necessary bones from those who had lived before in another world,the 4th sun ( there is a hierarchy of worlds in Aztec beliefs).
He requested the help of Mictlantecuhtli, he is a deity associated with the owl, spider & bat. He is in charge of the lowest underworld. At first Mictlantecuhtli decided that he would help out with the job of collecting bones for Quetzalcoatl, but then when it came time for the bones to be collected he had a change of heart. Quetzalcoatl managed to obtain the bones and escape with them, in his haste he dropped some,when the bones fell the broke & this why human beings have different sized bones.