Final answer:
Ponyboy is most likely referring to a hobby or interest when he mentions outgrowing something and getting back with popcorn. This is a metaphor for moving on from the current situation or activity.
Step-by-step explanation:
Ponyboy is most likely referring to a hobby or interest when he says, 'I'll probably outgrow it. We'd better buy some popcorn and get back.' In this context, Ponyboy is suggesting that he will eventually lose interest in something and move on. Buying popcorn and returning is a metaphor for moving on from the current situation or activity. It is common for people to outgrow their hobbies or interests as they grow older and their priorities change.