Panthers, cougars, or mountain lions, are actually not big cats but rather the worlds largest small cat and is more closely related to the domestic or house cat. The difference between big and small cats is simply if they can purr or roar. Big cats like lions, tigers, leopards, etc can roar but not purr (lions are the only big cat that can somewhat purr, they can only make the purr-like noise when breathing out though) where as small cats like your house cat, lynx, wild cats, panthers, etc can purr but not roar. This is due to the hyoid bone which connects the tongue to the roof of the mouth. In big cats this has an elastic segment which allows a big cat to roar where as in a small cat the segment is hard all over which makes it impossible to roar and creates a purring sound instead.