Q. If Israel was given all the land between the Euphrates and the River of Egypt, why do we find it occupying only the land west of the Jordan during the Millennium?
A. In Genesis 15:18-21 The Lord only specified the northern and southern boundaries of the Promised Land by naming 2 rivers. In the North is was to be the Euphrates which runs in an east/west direction before turning southward. Only the east/west portion was intended. In the south it was the river of Egypt (not the Nile but the Wadi Al Arish) . If you read Numbers 34:1-12 you can plot the four boundaries of the promised land and see that the western one was the Mediterranean and the eastern one was the Jordan River.
At the time of the conquest the Lord gave the tribes of Reuben, Gad and Manasseh permission to settle east of the Jordan opposite north and central Israel, but forbade the Israelites from taking any land He had given to Esau, Moab, and Ammon in the south. This eastern land will not be returned to Israel in the re-distribution that takes place at the beginning of the Millennium.