They’re nuts. A big male polar bear weights in at between 1200 and 1500 pounds. Even killer whales (the utter top predator) don’t take on the big ones. A lion or tiger? Max 750 or so. It would be an ass kicking.
It’s not a match of mass alone. It’s agility, the ability to impart momentum, properties of the claws and fangs and overall intelligence. The tiger comes out ahead because of its ability to attack from disparate directions.