Example sentences from the web:

• As long as you stay, you’re a big fish in a small pond… …and can blame everyone for holding you back.
• 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.
• I’ve got to get out of this town. It’s just too small. I’m tired of being a big fish in a small pond. I want to move to a big city like New York or Los Angeles.

NOTICE that this idiom could be used with a different meaning, referring to someone who is important in a small group or organization, but who won’t be so important in a larger one.

• “If he is such a great actor, why doesn’t he move to New York City?”
  Reply: “He likes being a big fish in a small pond.”

This idiom is explained very well here: http://painintheenglish.com/ 

“For example, a man started his own heating / air conditioning repair company. He had 4 locations and a fleet of 10 trucks. He was a prominent businessman in the community and his business netted $1 million per year. In a town of 25,000 people, he was in the top 1% of incomes and because he brought so much money into the community, he had a certain leverage with the city council. They would be very careful to allow building permits near his businesses without checking with him first.

However, if that man moved to a large city, for example: New York City, then 4 locations, 10 trucks, $1 million / year would not be special at all. For a small town (pond), he had a big business (he was a big fish), but in a large lake (New York City), he was a much smaller business (much smaller fish) by comparison”.