This would be a female bear, as she is with her cubs. Polar bears are actually the second most sexually dimorphic mammal. Consequently males are vastly bigger than females.

The most sexually dimorphic? The elephant seal. Those dudes are immense.

Edit: this, I read on wikipedia a couple of years ago. The information is no longer there but the fact that polar bears and elephant seals are among the most dimorphic mammals remains. The precise ranking is perhaps a little more obscure. https://en.wikipedia.org/wiki/Polar_bear