http://www.thunderridgebison.com/bison_history.htm ~ this guy has your answer. North American bison are descended from the European bison ~ which is a smaller animal. With the destruction of the large cats the buffalo simply “evolved’ into the large grazing animal niche and became much larger. Did you know these guys can jump 6 foot straight up? This is like your car jumping 6 ft in the air ~ makes it hard to hunt them efficiently without the use of horses or helicopters!
They had a very efficient way to hunt them - they just drove herds of them off of cliffs.
I had no idea buffalo had that kind of vertical leap though.