The bigger the storm, the more expensive the big fish.

The bigger the storm, the more expensive the fish. The next sentence is:

Natural selection, survival of the fittest, but don't be a sailor if you can get by on land.

The bigger the storm, the more expensive the fish. It means that it is very difficult to fish in stormy weather, and fewer fish can be caught. Things are scarce, so fish will be more expensive, that is, the greater the market fluctuation, the higher the risk and the higher the income, that is, the "big fish" will be caught.