Or HaChaim on Genesis 13:15/אור החיים על בראשית י״ג:ט״ו

1 כי את כל הארץ, For the entire land, etc. We are all familiar with the limitations of our eyesight. We can see for a certain distance, no more. We are also familiar with the so-called קפיצת הדרך, telescoping a distance of earth under one into a very small distance such as was experienced by Eliezer on his way to Charan, and Jacob on his way to Charan. In this instance G'd brought the various borders of the land of Israel closer to Abraham so that he could see them all with his normal eyesight. Abraham took possession of the land symbolically because all of it was within his view. Normally speaking, there is no more absolute sign of possession than to move a rock from its location. Here G'd moved the entire land from its location to bring it within the sight of Abraham's eyes.