Commentary for Genesis 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.

The man prospered. Scripture attributes this prosperity to Yitzchok’s own merit rather than to the bounty of the land. He continued to prosper. He never suffered a setback, which demonstrated that his prosperity was not by chance. Until he became very great. If his prosperity had been the result of favorable circumstances it would have reached a limit and then been followed by a downturn.