I haven't seen as many Walkers as Fox Trotters, but Fox Trotters vary ALOT in conformation too.
Mine is broad in the chest and kind of looks like a stock horse cross with not overly muscular hindquarters. (Or a giant Arabian as I like to think of her!). But my friend has 3 Fox Trotters and none of them look like each other or like mine at all. One is built like your guys Walking horses. Kind of tall and lanky. One is all around small and dainty with fine bones and just all around petite. One is sort of inbetween and kind of goose rumped. Not one looks alike!
She also used to own a Walker that was really different. Short, stout and round. Like an old-type Morgan.
I am always amazed at how gaited horses can look so different and yet still have the same gaits. There doesn't seem to be a fixed type in the breeds. Only the gaits and ancestory are in common.