A staple motto of the American muscle car crowd. It means that engine size (displacement) is the ultimate horsepower producing factor of a car, and that other horsepower-increasing devices, such as turbochargers can never fully substitute.
This motto has sound basis. An engine with a small displacement can be modified by car manufacturers to have a relatively large horsepower in several ways, the most important being cam modifications.
Cam modifications shift the torque curve in a certain direction; in this case, toward the high-end range. Since power is proportional to torque times speed, torque produced at a higher RPM level will yield more power. This type of cam modification is used religiously on Japanese cars, allowing cars like the Toyota Celica to get 150+ horsepower out of a Geo Metro sized engine. However, there is a catch. This type of cam modification means the engine will only start making sufficient torque and horsepower at very high RPM's. For example, the Celica makes peak torque at around 6,000 RPM and peak power at around 7,500 RPM. Torque is the turning force that allows cars to accelerate. The more torque, the faster the car accelerates. To have to rev up to 6,000 RPM (which is the engine redline of most cars!) just to get sufficient torque for a fast launch would take a long time, and the massive amount of high-pitched noise that is produced at such a high RPM level hinders any pleasurable experience one may have from driving the car.
An engine with a large displacement, however, has enough horsepower being produced by merit of its size that no radical cam modifications are needed. Therefore, there is much more low-end torque, meaning that the torque is readily available at commonly-reached RPM levels. The Chevrolet Camaros of old, with their 300+ cid displacement, had a torque peak around 2500 RPM!