Definition of big girl:
(n) :
(idiomatic, informal) An adult female.
(n) :
(informal) An older girl or grown-up girl; a girl who is no longer an infant.
(n) :
A female supposed to behave as a responsible adult.
(n) :
A woman who is overweight.
(n) :
(informal, derogatory) A timid, weak or effeminate boy or man.
(n) :
Used other than figuratively or idiomatically: see big, girl.