Uncle Jack's admonition that Scout is "growing out of her pants" is not to be quite taken literally: It is similar to the old idiom that someone is "getting too big for their britches" (or breeches). It simply means that a person (usually a child or youth) is growing too fast for their own good, and it implies conceit, cockiness or an attitude not quite befitting of one's age or position. In Scout's case, her cursing has Uncle Jack concerned, and she seems to be using questionable language at every turn; she is growing up but much too fast. Of course Scout is deliberately testing Atticus and the other adults since
I was proceeding on the dim theory, aside from innate attractiveness of such words, that if Atticus had discovered that I picked them up at school he wouldn't make me go. (Chapter 9)
Interestingly, the term "too big for his breeches" was first coined by none other than the legendary American frontiersman, Davy Crockett when describing his old friend, President Andrew Jackson.
It is first found in print in An Account of Col. Crockett's Tour to the North and Down East, 1835, written by Davy Crockett.
I myself was one of the first to fire a gun under Andrew Jackson. I helped to give him all his glory. But I liked him well once: but when a man gets too big for his breeches, I say Good bye.