Are we talking a Great Lakes strain of smallmouth at 20"?. Big water smallies are quite different than inland, smaller waterbody fish. They grow, feed, migrate and the genetics make them almost a different sub species. Just like the walleyes and perch, pike and so on. A 20"er from a Great Lake and a 20" smallmouth from a Kawartha type lake probably have a 5+ year age and a pound in weight seperation.