“Shammai said, make your Torah study fixed, say little and do much, and receive everyone with a cheerful countenance.” (Pirkei Avot)
Tzaddikim promise little and do much, while the wicked make big promises and do not even keep a minimum.
All Avraham promised the angels was a little bread, but subsquently he served them a meal fit for a king for which he slaughtered three oxen and used nine sa'ah of flour. He also served them butter and milk. (Bereishit 18)
How do the tzaddikim know that one must promise little and do more? They imitate Hashem Himself. When He promised that He would judge the Egyptians as the end of the exile. He assured Avraham, "And also the nation whom they shall serve will I judge!" (Bereishit 15:14) The promise contained merely two letters - דן (I will judge) yet subsequently He brought Ten Plagues upon the Egyptians!
