Lets use some logic. First, forage for smallmouth is a finite resource unless there is constant precipitation keeping the water high. When the river gets low the smallmouth have less food to eat and in a pure catch and release area there will be more fish. It seems easy to understand that if there are more fish in the same area that the fish will have less food and therefore be smaller. The thing to remember is smallmouth are not like trout, being that they do not travel miles up or downstream to find food. They usuall stay within 100yds of where they are born(this is according to a book I read over 10 years ago so whether or not it can be trusted is up to you).
As to the "muddy hole in the ground" I will say that when it comes to forage entering the pond it works much like the river. We have a 2.5 acre pond purpose built for bass fishing. 15ft max depth, trees, moss, and a wet weather creek that feeds it. We stocked 400 crappie, then the next year stocked 100 florida strain largemouth bass. Other than minnows nothing else has been put in. Within 4 years of stocking the pond we have caught numerous 6-8lb largemouths. Why? Because there are more predators(largemouth) than prey(crappie). The same principle applies to the river.
500 years ago I was not alive to know how big smallmouth bass in the buffalo river were, but I would bet they are the same or smaller than now due to overpopulation.