It is absolutely a function of stack length, or more precisely a function of the length of the intake tract, from the inlet opening of the runner to the intake valve. The length of the intake tract will determine the sonic tuning function, and leads to the peaks and valleys in the torque characteristic of an individual runner intake.
The short stacks I used on the SOHC street setup at Drag Week were only short because I needed them to fit under the hood, they weren't designed that way for any performance reason.