Piano
Piano
[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?
For the record, pianist /'piənɪst/ and penis /'pinɪs/ are near homophones in English. This is because pianist has a different stress pattern than piano /pi'æno/. The vagaries of the English language.

This comic is the reverse of the "twelve inch pianist" joke that appeared in the rec.humor.funny Usenet newsgroup .
In the original joke, a man found a genie in a bottle, and it is implied that he wished for a "twelve inch penis ," but the genie misheard him and instead granted him a "twelve inch pianist ."  The crux is that the word pianist sounds similar to the word penis ; the joke also relies on the common trope of genies granting wishes (as in the tale of Aladdin and the Wonderful Lamp ), sometimes with creative (mis)understanding or literalism as an Aesop to be wary about what you wish for, as well as the common stereotype that larger penises are more desirable.
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 has understood this implication, and therefore wants to have sex with Cueball (in accordance with the aforementioned stereotype).
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 , making him less desirable according to the stereotype.
Genies (or the magic lamps containing them) are mentioned in at least four other comics:
[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?
For the record, pianist /'piənɪst/ and penis /'pinɪs/ are near homophones in English. This is because pianist has a different stress pattern than piano /pi'æno/. The vagaries of the English language.
