In my experience, the opposite is true. Consider a farm pond that can support 1000 pounds of fish. They can be 10000 fish weighing a tenth of a pound or 100 ten-pounders. In this instance, the more heavily fished the pond is, the larger the fish will be.

Natural selection. The smaller the fish, the less likelihood that it would get caught. Fishermen do tend to favor bigger game. So the fishermen has routed out the large fish populations leaving the smaller varieties to propagate more.

But then there maybe other explanations for this like change in the water composition, temperature, introduction of other biological components, etc… Complete facts has to be collected and presented to arrive at any workable hypothesis. Simply correlating the size of the fish and the extent of fishing practices may lead to conclusions that may be erroneous and misleading.

Hope that helps. Cheers.