What do we learn from "Ish k'Achiv"?
Rashi: It will not be like initially, "Ish l'Fi Fekudav" (Bamidbar 26:54; each Shevet received according to its population 1 ). Rather, now each Shevet will receive equally, like rows in a vineyard (they are parallel and spaced evenly). Each portion will be from the eastern border until the Okainus ocean (refer to 47:8:3:1**) in the west. Two Shevatim will not be in the same strip.
Malbim: The Sifri holds that each Shevet received a strip 75 Mil wide. I say that there will not be a border to the length; a large Shevet will extend its border in the east and west.
Radak: Initially, Yehudah received a big portion, for it had many people.
Why does it say "Asher Nasasi Es Yadi"?
Malbim: This will be in the merit of My Shevu'ah to the Avos. I already promised to Avraham [to include the land of] Kini, Kenizi and Kadmoni (Bereishis 15:19), and to Yakov an inheritance without borders 1 .
He told him "u'Faratzta Yamah va'Kedmah v'Tzafonah va'Negbah" (Bereishis 28:14. Here Malbim implies that there will be fixed borders in the north and south ? refer to 47:14:1:2! - PF)