From what I've researched, unarmed strikes are natural weapons even if natural weapons aren't unarmed strikes. Can a monk use Morphic Weapons to increase the size category of unarmed strikes? Emphasis mine:
Morphic Weapons (Su): As a move action, a warshaper can grow natural weapons such as claws or fangs, allowing a natural attack that deals the appropriate amount of damage according to the size of the new form (see Table 5—1 on page 296 of the Monster Manual). These morphic weapons need not be natural weapons that the creature already possesses. For example, a warshaper polymorphed into an ettin (Large giant) could grow a claw that deals 1d6 points of damage, or horns for a gore attack that deals 1d8 points of damage.
If the warshaper's form already has a natural weapon of that type, the weapon deals damage as if it were one category larger.