I would say spontanious bantam. I have a little araucana pullet same age as yours and half the size of the other birds her age. She is very healthy, always the first to the food dish, chipper and happy. She is just not going to get very big and its a pain because so far she is my nicest looking baby this spring and I don't do bantams.
Bantams do pop up in large fowl every once in a while. There is an actual term for it but I am way too tired to remember.