Avraham's service rooted in his children

ואעשך לגוי גדול ז"ש אלקי אברהם כו' פי' שבנ"י ממשיכין מלכותו ית' עליהם בכח קבלת מלכות שמים של האבות ז"ש ואעשך כו'

"And I will make you into a great nation" — this is the meaning of "the God of Avraham, etc." The explanation is that Bnei Yisrael draw down His malchus (kingship) upon themselves through the power of the Avos' acceptance of malchus Shomayim (the yoke of Heaven). This is the meaning of "And I will make you, etc."

The Sefas Emes reads the blessing to Avraham as the source of how every Jew is able to take on the yoke of Heaven. Because the Avos accepted Hashem's kingship completely, their descendants inherit the power to bring that kingship down into their own lives.

כי אברהם אע"ה הי' עובד להשי"ת בכל כחו אשר כביכול לאין שיעור לשלם שכר והי' זה השכר אשר כל עבודתו נתפשט בכל בנ"י בניו לדורותיהם וכל תנועה ועובדא שלו נעשה שורש לבניו בנ"י וכ"כ במד' ברכתו כעץ שתול על פלגי מים משל לרואה אילן יפה כו' ואומר במה אברכך רק שכל ענפים שיצאו מאתך יהיו כמוך כו' ע"ש.

For Avraham Avinu, peace be upon him, served Hashem with all his strength, so much so that, as it were, there was no measure with which to repay him. And this was the reward: that all of his avodah (Divine service) spread out into all of Bnei Yisrael, his children throughout the generations, and every movement and deed of his became a root for his children, Bnei Yisrael. So too it is written in the Midrash regarding his blessing, "like a tree planted by streams of water" — a parable of one who sees a beautiful tree, etc., and says, "With what shall I bless you? Only that all the branches that come from you should be like you, etc." See there.

Avraham's service was so boundless that no ordinary reward could match it. So Hashem's "payment" was that Avraham's avodah would live on in his descendants — every act of his became a root from which the whole nation grows, the way a planted tree's blessing is that its branches resemble it.

וז"ש ואעשך לגוי גדול שנתפשט כחו לגוי גדול כנ"ל כי הקב"ה משלם לכל ברי' שכרו מה שחביב אצלו יותר.

And this is the meaning of "And I will make you into a great nation" — that his power spread out into a great nation, as above. For the Holy One, blessed is He, repays every creature with the reward that is most precious to it.

Hashem does not reward a person with something generic, but with whatever that person cherishes most. The deepest reward fits the deepest desire of the one being rewarded.

ובאברהם אע"ה הי' זה שכר היותר חביב שיהי' ממנו גוי גדול אשר המה מיוחדים לעבודתו ית' והי' זה שכרו כנ"ל:

And for Avraham Avinu, peace be upon him, this was the most precious reward — that from him there would come a great nation who are dedicated to Hashem's service; and this was his reward, as above.

What Avraham wanted more than anything was not personal honor but that Hashem be served. So the perfect reward for him was an entire nation devoted to that very avodah.

Summary: Avraham's boundless service to Hashem could not be repaid by any ordinary measure, so Hashem's reward was to extend Avraham's avodah into an entire nation rooted in him — Bnei Yisrael, who carry forward the Avos' acceptance of the yoke of Heaven, which was the reward most precious to Avraham himself.