Which big cat is not in the genus Panthera? The answer depends on how you define ‘big cat’. Historically the big cats are the cats within the genus Panthera. That’s the whole point. These big cats are the classic foursome, all of which can roar: tiger, lion, jaguar and leopard. However some people include the snow leopard in this group. They are also of the genus Panthera but the snow leopard cannot roar. It is a different kind of animal: less aggressive towards people.
If by ‘big cat’ you mean any large cat then you would include the other large cats which are the puma, cheetah, clouded leopard (medium-large) and Sunda clouded leopard (medium-large). These cats are not in the genus Panthera. Respectively they are in the genus: Puma, Acinonyx, Neofelis and Neofelis. You might add the serval but once again I call this cat medium-sized or medium-large. This is genus: Leptailurus.
As the question in the title refers to a single big cat not in the genus Panthera, I’d have to select the puma (aka cougar) which is the largest of the group referred to.
Indeed the puma is slightly larger than the leopard.
