big frog in a small pond

big frog in a small pond  {n. phr.},  {informal}

An important person in a small place or position; someone who is respected and honored in a small company, school, or city; a leader in a small group.

As company president, he had been a big frog in a small pond, but he was not so important as a new congressman in Washington.

Contrast: LITTLE FROG IN A BIG POND.