Statement heard commonly on the streets in my neighborhod whenever a person roars down the street in a muscle car or a very large pickup, or sometimes even a really nice sports car. The theory behind it goes something like this:
The men in these vehicles have small penises so they compensate for it by having big or really nice cars.

The (more likely) reality of this:
The person saying this wishes he had a car/truck like that and wants to feel better about not having one by claiming that the person driving it is just compensating for a small peins.

If the former is true then I must have a huge penis as I have no car. I wonder if getting rid of my car caused my penis to grow.