In Chapter 9, Uncle Jack tells Scout that she is growing out of her pants. What does this mean and why might he say it?

In the book, To Kill a Mockingbird, Uncle Jack tells Scout that she is 'growing out of her pants' most likely for two reasons. First, he is referencing the fact that she's growing up (no longer a little girl any more), and secondly, it could be a euphemism for not 'knowing her place' and talking back to the adults in the room.