the.com/big fish

means An important or powerful person — but usually only within a small, limited pond, not the wider ocean.

from Straightforward nautical-flavored English: fishermen have long sized their catch and themselves by comparison — a 'big fish' is simply the largest, most impressive catch around. The phrase picked up its figurative sense of 'important person' by the 1800s, and the fuller idiom 'big fish in a small pond' followed to needle anyone whose importance turns out to be local rather than universal. No hidden root, no borrowed language — just old, plain metaphor that swam straight from the water into everyday speech.