In order to move through the air, a rocket has to push air aside. Things that are designed to travel quickly, such as sports cars and jets, are shaped to minimize the amount of air they push aside to move forward. Your paper rocket has been designed to move through the air efficiently, too. The V-shaped nose helps it cut through the air as it flies; the fins help keep the rocket moving in a straight line.
If you tried two different-length paper cylinders for your rocket experiments, you may have noticed that the longer rocket flew further/higher than the shorter one. This is because a long rocket body has more distance (length of rocket) over which the force of the air from the PVC tube is applied, which increases the amount of time during which the force is applied. As the air expands into the pipe and rocket, however, the force decreases, so there is some optimal length of rocket where force and time are maximized.
The launching force is created by stomping on the 2-liter bottle. Increasing that force will make the rocket go higher/further. The change in velocity is proportional to how much force you apply and the time during which the force is applied. The change in velocity is inversely proportional to the mass of the rocket. Therefore, lighter rockets tend to go further/higher.
These variables—the force of the stomp, the length of time during which the force is applied to the rocket (which, in this case, translates to the length of the rocket), and the mass of the rocket—all affect how high and far the rocket flies.