a big fish in a small pond
US /ə bɪɡ fɪʃ ɪn ə smɔl pɑnd/
UK /ə bɪɡ fɪʃ ɪn ə smɔl pɑnd/

1.
a person who is important in a small group or organization but who would have much less importance in a larger, more competitive group or organization
:
•
He was a big fish in a small pond at his old company, but now he's just one of many at the new multinational corporation.
•
She prefers to be a big fish in a small pond, where her contributions are highly valued and recognized.