"And it is a little one" (Genesis 19:20). For its settlement was not equal to that of Sodom, since the settlement of Sodom was fifty-two years, and the settlement of Tzoar was fifty-one years, as it is said, "let me escape (na) there" (Genesis 19:20) — and "na" in gematria is fifty-one. Therefore it was called Tzoar, and it was younger [tze'irah] in its settlement and in its iniquities.
Why Tzoar Was Spared and Named the Little One
Midrash Aggadah, Genesis 19:20
והיא מצער. שלא היתה ישיבתה שוה לסדום, שישיבתה של סדום נ"ב שנה, וישיבתה של צער נ"א שנה, שנאמר אמלט נא שמה, ונא בגימטריא אחד וחמשים, לכך נקראת צער והיתה צעירה בישיבתה ובעוונתיה: