Good thing he didn't make it smaller, or it'd need someone three inches tall to play it.
This comic is the reverse of the old "twelve inch pianist" joke. In the original joke, a man presents a miniature piano, which is played by man of proportionate size. When asked about it, he reveals that he found a bottle with a genie inside, but the genie misunderstood his wish, and gave him a 12-inch pianist. The crux of the joke is that the word pianist sounds similar to the word penis. The man considers having a larger member to be desirable (presumably due to the stereotype that it will make him more sexually appealing), but loses out on the opportunity, due to the misunderstanding.
In this comic, the opposite happens: Cueball wished for a "twelve inch pianist" to play his miniature piano, but was misheard by the genie, implying that the genie instead granted him a "twelve inch penis." Megan quickly grasps the implication, and therefore wants to have sex with Cueball (in accordance with the aforementioned stereotype; and also judging from the weird tone in "...are you").
The title text suggests that if Cueball had made a smaller piano – and had thus wished for a smaller pianist – he would have instead received a smaller penis.
Genies (or the magic lamps containing them) are mentioned in at least five other comics:
- 152: Hamster Ball
- 879: Lamp (which is also related to penises)
- 1391: Darkness
- 2193: Well-Ordering Principle
- 2741: Wish Interpretation