The polar bear is 3x heavier — the lion is outclassed by sheer mass.
The polar bear outweighs the lion by roughly 3:1 (1,500 lbs vs 500 lbs) and has comparable weaponry. This size advantage is decisive in a fight between two large predators.
The lion's advantages — speed, agility, and combat experience with large predators — are real but insufficient against something three times its weight with 4-inch claws and a devastating bite.
The lion's mane provides some protection against throat attacks, but the polar bear's fighting style relies more on powerful paw swipes than throat bites. A single full-force swipe from a polar bear can decapitate a seal — it would be devastating to a lion.
The lion is faster with more combat experience. Its mane protects against throat attacks. Lions regularly fight other large predators and have a warrior's mentality.
3:1 weight advantage. Larger paws with longer claws. Comparable bite force. Thick fur and fat provide natural armor. The size gap is the same as a middleweight vs a super-heavyweight.
The polar bear wins through overwhelming size advantage. The lion is a magnificent fighter, but the 3:1 weight differential is too extreme.
African Lion also fights
Polar Bear also fights