big fish in a small pond
Also, big frog in a little pond. A person who is important in a limited arena; someone overqualified for a position or in relation to colleagues. For example, Steve has both a Ph.D. and an M.D., yet he's content with his practice at a rural hospital; he prefers to be a big fish in a little pond. The expression big fish has been slang for an important or influential person since the early 1800s. The addition of in a small pond as a metaphor for an unimportant organization is more recent, as is the substitution of frog. Another variant is the proverb Better a big fish in a little puddle than a little fish in a big puddle.
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