The weight of a polar bear changes as it goes through its life. As a polar bear gets older and grows into an adult its weight increases
The average weight of a female polar bear is usually between 150-250 kg. While the average weight of a male polar bear is usually between 350-700 kg. Seeing as the weight of the average small family car is said to be around 1000kg, a polar bear weighing 700kg is quite a weight. The heaviest recorded polar bear weight actually weighed in at around 1002kg, so the same as the average small family car! However the average weight of polar bears in general is said to be 450 kg. These weights vary not only on the age of the polar bear but also during the year.
When polar bears are first born they often weigh 0.9 kg. While inside the den with their mother they usually grow another 9 to 14 kg and so leave the den and enter the outside world with a weight of 10 to 15 kg.
Polar bears hunt during one part of the year and often fast and go into a period of walking hibernation during another part of the year and so their weight varies. Polar bears will be heaviest just after the hunting season and will be lighter after the period of fasting just before the beginning of the next hunting season.
The second heaviest bear in the world is the brown bear. Brown bears are found to have an average female weight of between 130-220kg and an average male weight of between 265-355kg. Brown bears are found in warmer environments than polar bears and are located in multiple places in northern America and northern Eurasia. However one subspecies of the brown bear known as the Kodiak bear does rival the polar bear in size and weight.
You can read more about polar bears here
Also check out polar bear endangered