That's a plausable assumption. Yes, muscle weighs more than fat, but you have to consider the fact that river fish consist of lean muscle mass, that in no way compares to the overall amount of fat that lake fish can have. Its not like you see river fish with huge, bulging muscles. That's like comparing a man from Ethiopia to an overweight American. Yeah the Ethiopian is all muscle, but it's not enough to compare to the weight of the fat American. I guess if you compare one fish from a lake and one from a creek with the same length and girth, the one from the creek would probably weigh more, but if you find a lake fish that looks like your run of the mill creek fish, he probably isn't very healthy, and the lake fish of the same size would also probably be younger. Lake fish have shad and other larger forage to feed on. Shad can REALLY put some weight on black bass. Also, shad and other larger forage are very rich in protein, so they will still gain some muscle as well. Crayfish is a favorite of black bass, but it doesn't fatten them up like shad, giving the lake fish the biological upper hand when it comes to potential or average size.