John Gill Commentary Deuteronomy 19:8

"And if Jehovah thy God enlarge thy border, as he hath sworn unto thy fathers, and give thee all the land which he promised to give unto thy fathers;" — Deuteronomy 19:8 (ASV)

And if the Lord your God enlarge your coast
Extend it further than it was upon their first settlement, even carry it as far as the river Euphrates, as in the times of Solomon, (1 Kings 4:21 1 Kings 4:24) . Jarchi interprets it of such an enlargement as to give them the land of the Kenites, the Kenizzites and Kadmonites:

(as he has sworn unto your fathers), and give you all the land which
he promised to give unto your, fathers :
Abraham, Isaac, and Jacob; see (Genesis 15:19) (Genesis 26:3Genesis 26:4) (Genesis 28:13Genesis 28:14) .