DIRECTLY TAKEN FR0M A Z000L0GY B00K THAT I 0WN.
A polar bear, unlike it's Alaskan cousin, is adapted almost completely for a carnivorous diet, as vegetable matter is very, very hard to come by in it's habitat. This can be evidenced by the Bear's sharper claws and it's build, which is more suited to taking down prey as large as a Walrus, which mind you a male of this species is capable of possessing a mass of over 4,400 imperial pounds and is protected by very thick skin and an even thicker layer of blubber/fat. Also, Walrus's aren't easy prey, those Tusks they have are more than capable of killing any land carnivore on earth with one blow, and Walrus's are easily able to crush the bear to death, not to mention that the Walrus has sheer size protecting it.
Kodiak bears eat far more insect, fish, and plant matter than Polar bears and hunt a lot less, and their adaptations show this. Their claws are blunter, their teeth are not quite as good for killing, and their arms are not quite as well designed for grappling with large prey.
YUP P0LAR BEARS WITH THEIR BETTER EYE SIGHT, LARGER SLENDER NECKS AND L0NGER ARMS, SHARPER CLAWS AND CANINES MAKES IT AN EASY FIGHT. I L0VE BEING RIGHT.