What does "big girl" mean?

[waterrunsfast]

A song named "big big world", has a word "big girl" in it. I don't know what's the meaning of it. Could someone tell me? Thanks!

[Tdol]

It normally is used to mean that someone has grown up and is no loner a little child. It can also be used as a way of cricising someone for behaving in an immature way.

[waterrunsfast]

Thank you! I know now.
I used to think "big girl" may mean "a girl who is strong in spirit", because the song repeated "I'm a big big girl in a big big world, it's not a big big thing that you leave me".
By the way, does "big thing" here mean something important?