What evolutionary factors can contribute to large sexual dimorphism in large mammals?
On Earth, the polar bear is among the most sexually dimorphic animals, with males being on average almost twice as heavy as females (Wikipedia claims average weights of 450 kg for males versus 260 kg for females, with a significantly larger difference in the extreme cases).
There is an obvious natural limit to how large dimorphism can be sustained, namely the reproductive behavior of the species. In mammals, if the male is unable to safely mount the female, then procreation becomes much more difficult and at the extreme end impossible. That would then lead to either limit or reduce the dimorphism in the species (possibly instead leading to a subspeciation, with size as a determining factor), or lead to a change in the reproductive behavior of the species.
However, what if we are looking at it from the other end of the scale? What evolutionary factors could contribute to sexual dimorphism in animals, and what factors may contribute to supporting a large degree of sexual dimorphism in large mammals?
Bonus questions: Does the polar bear represent approximately the largest sexual dimorphism sustainable in large mammals in an Earth-like environment, and if so what might be the specific limiting factor in the case of that species? Would a larger degree of dimorphism be sustainable in an environment different from the polar bear's natural habitat, and what environmental difference might be the determining factor in that case?
For the purposes of this question, I am primarily concerned with differences in size, but if answers touch on other aspects as well I certainly am not going to downvote because of that.
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