Title text: Good thing he didn't make it smaller, or it'd need someone three inches tall to play it.
The joke here is the reverse of an old joke that appeared in rec.humor.funny. In the original, the story-teller reports that he made a wish with a hard-of-hearing genie for a twelve inch penis and instead got a twelve inch pianist. In this version, Cueball wished for the twelve inch pianist to hard-of-hearing genie, and instead received a twelve-inch penis, to Megan's interest.
The title text is saying that if Cueball had made a smaller miniature piano, he might have wished for a smaller "pianist" to play it and received an unfortunate size reduction.
- [Cueball is holding a box with an open lid. A miniature piano is inside. Megan is looking at it]
- Cueball: My hobby is making miniatures. Check this out — it's a fully-functional grand piano.
- Megan: Woah — beautiful.
- [Cueball looks at the miniature piano]
- Cueball: Sadly, I've never heard what proper music sounds like on it—the keys are too small to play.
- [Cueball closes lid to the piano.]
- Cueball: I once asked a genie for someone who could play it for me, but I think he misheard.
- Megan: ... are you doing anything later?