“Unknowable” has pointed out that there is a cat even bigger than the American lion was: the liger, the offspring of a male lion and a female tiger. These hybrids are sterile, which is probably a very good thing. We wouldn’t want them devouring all the elephants.
I, Mr. Nature, stand corrected. I knew about these critters but wasn’t counting them because they can’t reproduce themselves. There’s also a tiglon, with a tiger father and a lion mother: not quite so big, but somewhat more aggressive.
Nature sure is full of surprises.