פּירוש אויף בראשית 26:13

כי גדל מאד UNTIL HE BECAME VERY GREAT — so that people used to say, “Rather the dung of Isaac’s mules than all Abimelech’s gold and silver’’(Genesis Rabbah 64:7).

ויגדל, in material wealth and all types of possessions.

So that people would say: Rather the dung of Yitzchok’s mules... Otherwise why does it say, “Until he became very great”? [The next verse details his wealth! Perforce,] it comes to tell us the extent [of his blessing]. Thus Rashi mentions mules, since mules [ordinarily] have no blessing at all, as Rashi explained in Devarim 15:14.