Little in this case is used with miss to give the nickname little Miss New Boobs to Ross' new girlfriend. Little miss (something) is a nickname given to a woman you don't like or that you think is pretentious and snobby. This title has a very sarcastic nuance and the word little belittles the title of the person and makes them appear small and insignificant.
Ex: Here comes little Miss Perfect.
Ex: Oh look, little Miss Good Grades is bragging about her achievements again.