See: LITTLE 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.