big frog in a small pond：小地方的要人：

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

作为公司的总裁，他在这个小地方一直是个受人尊敬的人；但在华盛顿，作为一个新当选的众议员，他却算不上什么。