The Gemara relates –
Rabbi Yoḥanan said in the name of Rabbi Yosei: With regard to anyone who delights in the Shabbat, God gives him a boundless portion, i.e., a very large reward, as it is stated: “If you keep your feet from violating Shabbat, from pursuing your affairs on My holy day, and you call Shabbat a delight, the Lord’s holy day honored, and you honor it by not going your own way, or attending to your own matters or speaking idle words. Then you shall delight in the Lord and I will cause you to ride on the heights of the world, and to feast on the inheritance of Jacob your father, as the mouth of God has spoken” (Yeshayahu 58:13-14).
The reward for delighting in Shabbat is specifically the portion of Jacob.
Not that of Abraham, about whom it is written, “Rise, walk through the land through its length and its width because I have given it to you” (Bereshit 13:17) i.e., only this land alone in its borders. And not that of Isaac, about whom it is written, “Dwell in this land and I will be with you and I will bless you because I will give all of these lands to you and your offspring” (Bereshit 16:3), meaning these lands and no others. Rather, that of Jacob, about whom it is written, “And your offspring will be like the dust of the earth, and you will spread out to the west and to the east and to the north and to the south, and all of the families of the land will be blessed through you and your offspring” (Bereshit 28:14).
There are no boundaries for Jacob’s portion.
The reward of a boundless portion is one that is measure for measure. Since one delights in Shabbat without limiting his expenditures, one is rewarded with a portion that is limitless (Beit Yosef). The Gemara citation of the verse from Yeshayahu speaks of one who delights in Shabbat as being rewarded with the portion of Jacob. Some commentaries explain that the portion of Abraham was shared with the descendants of Ishmael and Lot, and part of Isaac’s portion was given to Esau. However, the portion of Jacob was complete, not shared with others, and therefore boundless (Maharsha; Imrei Emet).