In my own words - The ideal environment for lions would be a vast lush open savanna spotted with clusters of trees and plenty of lakes, rivers, and creeks. This grassland would be teaming with great herds of ungulates and other large herbivores. In such an ideal environment, a typical mature male lion would likely weigh about 425 pounds. Make no mistake - a 425 pound cat is a huge cat!
The ideal environment for tigers would be vast lush temperate green forest heavily populated with ungulates and other large herbivores. In such an ideal environment, typical mature male tiger would weigh roughly 500 pounds.
The ideal environment for brown bears would be a mixture of open forest, lush grassland, and mountains. The lakes and rivers would be teaming with fish. The land would be heavily populated with large herbivores. Besides the large ungulates, there would also be small animals such as ground squirrels. There would be a large variety of nuts, acorns, berries, and other desirable vegetation. There would also be a variety of brood insects including honey bees, ants, and termites. And let's not leave out other predator species such as wolves and big cats from which the great bear can usurp fresh kills.
Living in such an environment, a typical healthy mature male brown bear would weigh roughly 800 pounds. Finding the average weight of brown bears is a rather complicated undertaking. First, we must consider the season. When a brown bear goes into hibernation in late Autumn, he is roughly double his weight when he emerges from hibernation in early Spring. Therefore, we should always choose the Summer months when weighing brown bears. All weights given here for brown bears are their Summer weights.
Second, we must consider age. While a male lion or a male tiger is fully mature at 4 years old, a male brown bear is not fully mature until he is 9 years old. Therefore, when I say 'mature', I am referring to a bear no less than 9 years old.
The third factor is food availability. There is a tremendous size range among brown bears due to their diets and their various environments. Brown bears living in food-poor environments can average only within the 300 pound range. Such a bear is the barren ground grizzly of the Canadian tundra. Never-the-less, a barren ground grizzly was recently weighed in at 800 pounds. A similar brown bear is the Yukon grizzly of Alaska. Although normally in the same size range as the barren ground grizzly, the record grizzly killed in the Yukon weighed 948 pounds. These unusually large specimens obviously discovered some extraordinary food sources. Perhaps they became exceptional hunters of caribou or musk ox.
While Montana and Yellowstone in Wyoming are picturesque, such environments are harsh. Food availability for brown bears is little better than on the arctic tundra. Here, the average healthy mature male brown bear weighs roughly 500 pounds. In Montana, the record grizzly weighed 790 pounds. In Yellowstone, there are always a handful of bears weighing from 800 to 900 pounds. These big bears very likely routinely rob wolves of their kills. The heaviest grizzly on record in Wyoming weighed a whopping 1,120 pounds. He was a notorious dumpster thief.