All phrases are intended to be used based on the situation. Multiple may apply in any given situation.

Some times, the big fish, in the small pond, can bring about a meaningful change to lives they can influence. Other times, the small fix in a large pond can bring about systemic changes that larger fishes in that pond cannot.

The important piece here is to be a “fish”, the instrument of change, that helps improve the world, in their own way.

Context is important. As is interpreting the phrase. That is why it is said that every time you read the Gita (or any philosophical text), you will find things relevant to your “current” life, and hence new meaning.