Really, so where do the large ones come from? Bennett consistently produced big rainbows that were said to have grown up in the stream. I suspect it is more a case of protection. There was a time when you couldn't see the bottom under the Rock bridge, in front of the whistle, the Suzy or the Bluff hole, just to name a few. These hiding places are pretty well gone.
Your example of 30# browns and 18# "bows doesn't make sense to me. That could well be true, but if the Niangua can produce 6# browns, where are the 3# 'bows?