No-one seems to dispute the fact that fish are significantly smaller today than they were a couple of decades ago. It is evident in virtually any stock where size data exists.
Nobody catches any of the really big ones anymore - they re all fish stories from years ago.
There are a couple of theories, that may or may not explain the actual causes of slow growth in fish. One theory that seems to be fairly well accepted today is that slower growing fish are the result of size selective mortality associated with fishing. These are the genes that will be passed on to the next generation. Repeat this over several generations. The fish that will be able to pass on their genes are those that are not removed by fishing.