Debriefing
Take a look at the typical results for the Ponzo illusion experiment. Note that in the baseline condition, where no reference scene is presented, most participants perceive both bars to be the same length (perceived length of line A/line B= 100%). But when the observer is shown a scene with distance cues, bars placed higher up on the scene are perceived as longer. This is gauged by how big the observer makes line B to make it match line A and is shown as a percentage. Thus the observer is using the distance cues in the picture to infer the size of line A.
The Ponzo illusion involves the inference of size based on cues in a scene. The cues in the scene are so powerful that they cause the observer to be unable to replicate the actual image being projected onto their retina. The Ponzo illusion illustrates how dependent people are on scenic depth cues.