little frog in a big pond

little frog in a big pond or small frog in a big pond  {n. phr.}

An unimportant person in a large group or organization.

In a large company, even a fairly successful man is likely to feel like a little frog in a big pond.

When Bill transferred to a larger high school, he found himself a small frog in a big pond.

Contrast: BIG FROG IN A SMALL POND.