"and He will grant you mercy and multiply you." R. Gamliel Berebbi says: All who are merciful to others are accorded mercy by Heaven.
"as He swore to your forefathers": All is in the merit of your forefathers.
(Ibid. 19) "when you hearken to the voice of the L-rd your G-d to keep…": From here they said: If a man begins to hear a little, in the end, he merits hearing much. If he begins to hear, in the end he keeps.
"to keep all of His mitzvoth that I command you this day": that a "slight" mitzvah be as beloved by you as a "weighty" one.