coasting
Explanation
This comic riffs on the "coastline paradox," a real concept from fractal geometry. The coastline paradox states that the measured length of a coastline depends on the scale of measurement: as you use smaller and smaller units, the coastline's length approaches infinity because of its fractal-like irregularity.
A couple is in bed, and one partner asks the other about the size of something using various metrics. The answers are: length -- infinity; area -- infinity; volume -- quite small; and the other partner's measurements are "quite quite small." The joke is that the person is trying to use the coastline paradox as a form of dirty talk or sexual boasting -- implying that certain measurements are "infinite" when examined at fine enough resolution.
The caption confirms the gag: "My attempt at erotic usage of the coastline paradox narrowly failed." The humor comes from the absurdity of trying to apply a mathematical concept about measurement and fractals to a romantic or sexual context, and the deflating punchline that the trick didn't actually work.