The balloon must contain enough energy (compressive force of the balloon on the air inside, and the amount of air it contains) to move the mass of the car.
It is the opposing force of the air exiting the balloon that propels the car forward.
Newton’s 2nd Law of Motion: Acceleration is produced when a force acts on a mass. The greater the mass (of the object being accelerated) the greater the amount of force needed (to accelerate the object). Heavier objects require more force to move the same distance as lighter objects. A balloon car that is too big (massive) or a balloon that is too small for the car size will not produce much, if any, acceleration.
Newton’s 3rd Law of Motion: For every action there is an equal and opposite re-action. Or, whenever an object pushes another object it gets pushed back in the opposite direction equally hard. The air escaping from the balloon is not just dispersing in the air. The force propelling it out of the balloon must be matched by the force forward. It is this forward force that moves the car. Along with the second law, this shows why it takes a large amount of a very fast exiting gas to move a smaller but more massive body forward at a slower speed.
A very good tutorial is here:
Here's a fun, older post about this experiment: