Size of body parts corresponds to number of neurons in the brain. Illustrated by Joan M.K. Tycko.

You may think your brain dedicates a significant portion of itself to your sex organ.  Surprise!

A tongue in cheek excerpt from ” The Accidental Mind” by David J. Linden

Most people who look at the sensory homunculus [pic on the left] long enough will eventually stammer out something like, “Given how sensitive the genitals are, shouldn’t they be larger?”  …  One potential explanation for the size issue hinges on the need to be more precise when we say “sensitive to touch.”  The parts of the homunculus that have huge representations (such as hands, lips, and tongue) are not merely able to detect faint sensations but can also discriminate the location of these sensations very precisely.  … The genitals, while they can easily detect faint sensations, cannot accomplish tactile form perception.  In the spirit of old-fashioned natural philosophy, you can experiment with this at home. In this way, they are somewhat like the cornea of the eye: quite sensitive to faint sensations …, but without an ability to precisely locate those sensations.