Wow LOL
I'm gonna have a real hard time refuting you on this, because you appear to lack the basic knowledge of fisheries for me to be able to frame it for you. Unmanaged fishing pressure suppresses the size of fishes in a watershed. That's not even a supposition, it's one of the most basic tenants of fisheries management.
This is true, whether or not you choose to believe it. Constantine Rafinesque, the guy who literally named many of the plants and animals in America for the first time in the early 1800s, is widely considered a reliable authority. He did his surveys in the Midwest after the Louisiana Purchase. He described 6,700 new species of plant and animal. He published works on evolution 30 years before Darwin. This guy knew his stuff. If that man claimed that 250+ pound blues were not unusual in a time prior to commercial fishing, I'm inclined to believe him.
This isn't "just supposed sightings".