SJS, I don’t think it always works out well mathematically, because many polygamous populations tend to have a higher rate of women who die in childbirth. I think this offsets the higher mortality rate of men due to hard labor/ farming/ hunting accidents. Also, I think that in most polygamous societies men tend to marry women of childbearing age, not older widows.
Many men do marry women younger than they are, so they have a larger pool of women to choose from in an increasing population, but there are also more younger men who may want to marry these women. In fact one common practice in Mormon polygamous cults is to expel young men, so that the young women in the cult will be more willing to marry the cult elders.
I’ve read that societies with high polygamy rates tend to be unstable, because many young men, especially those without financial resources, have no chance to marry and are therefore more likely to engage in destabilizing behaviors.