"Bigfoot" refers to the lower unit. It's bigger overall and more robust, often with a lower ratio and capable of swinging a bigger prop. They're usually used on large, heavy, slow moving vessels, like pontoons, houseboats, sailboats, or as kicker motors. They're still around although they've changed the name to command thrust.
Otherwise, to my knowledge, they are identical to a non-bigfoot motor aside from the lower unit.