As mentioned, (besides the embellishing of size) there seems to be two classes of walleyes. Some are built like a Tomahawk Missle, long skinny tubes that don’t pack on the weight. Even in the Spring pre-spawn, they don’t have that dense body mass to give them extra couple pounds. Then there are the freaky fat ones with incredible body mass. Your absolutely right in looking at what it takes for girth to get you into that heavy class of fish. At 28-1/2″, my GUESS would be that you need greater than 18″ of girth plus the cavity filled with spawn to break the 10# mark. That is a huge girth for that length of fish. Not to say they don’t exist, because we know they do. But for a walleye to have a girth that is greater than 60% of its length is not common.