Devarim Rabbah (chapter 4) preserves a comment of Rabbi Yitzchak on the verse, "When the Lord your God shall enlarge your border, as He has promised you" (Deuteronomy 12:20). It is really a teaching about how appearances deceive.
"A scroll," said Rabbi Yitzchak, "lying rolled up on the table — no one can tell how long it is, or how broad. It looks like a small object. But when you unroll it, it speaks for itself and reveals its full extent.
"So it is with the Land of Israel. Most of its surface is folded — hills, mountains, valleys, ridges. The land is much larger than it looks, but the folds hide it. When the Holy One, blessed be He, levels it — as the prophet Isaiah foretold: 'Every valley shall be raised, and every mountain and hill shall be made low, and the crooked shall be made straight, and the rough places smooth' (Isaiah 40:4) — then the land itself will speak, and its true dimensions will stand revealed."
The promise to enlarge Israel's borders is not, in this reading, a promise of conquest. It is a promise of unfolding. The land God already gave is larger than the map can show. What the messianic age does is simply flatten the paper so the full scroll can be read.