Dave...this is one subject I do find very interesting.
My wife caught a 4 lb 13 oz. smally that measured about 19 1/2" and this year nailed a 5 lb. 9 oz.(same digital scales) and we pulled out the pictures and compared them and I swear the smaller fish looks much bigger...you'd swear it was 6 for sure if someone hadn't said so.
Caught a 19" fish myself last year that was just shy of 5 and was by far the fattest smally I've caught and that includes the "footballs" I've caught at Erie...go figure....