"Small fish" means someone or something with little power, value, or rank, often in a bigger group. It contrasts with the "big fish," who have more sway or attention. It can also simply refer to a fish of small size.
I think in both of those situations, it's important as an actor to learn, despite the success I had as a kid, that it's important to understand what it means to be a small fish in a big pond. Anthony Michael Hall
The big fish run the show. I'm a middle fish, but there are small fish that are poisonous. Roberto Cavalli
The fast fish, not the big fish, eats the small fish. Ali Babacan
In the great river there are large and small fish. Ewe Proverb
Small fish are better than empty dishes. Faroese Proverb
In the great river there are large and small fish. African Proverb