The verse is saying that Israel showed gross ingratitude to God. When they “became fat” they forgot their Benefactor.
Why does the verse not use the usual name of Israel? After all, Yeshurun is from yashar, “upright”, and Israel are rebuked here for being the exact opposite of upright.
The choice of the name may be sarcastic: the supposed upright people are doing precisely what their name does not indicate. Hertz remarks that Yeshurun was intended as a title of honour. Instead it became a reproach.
Unfortunately the same can sometimes be said of other names. “Israel” is meant to denote the Prince of God, but sometimes the people of Israel were poor representatives of the Almighty.
“Judah” (from which comes the word “Jew”) is meant to indicate a people whom others would thank and acknowledge, but sometimes the people of this name acted unworthily.
“Hebrew” indicates “on the other side from the pagan nations”, but sometimes the forces of assimilation led Hebrews to adopt the least admirable aspects of non-Jewish cultures.
We are constantly reminded that it is hard to be a Jew, but the problem is not only because of the unfair attitudes of outsiders but because we do not always live up to the expectations and standards enshrined in the names we bear.
(It should be noted that “Yeshurun” often occurs in synagogue names; well-known examples are in Jerusalem, New York and Manchester).