If she chooses brighter males, then males get gaudy. If she chooses males which hang around and help bring up the young, then males get faithful and even helpful. These are the general patterns. In those circumstances where parental care is necessary to ensure the offsprings' survival, a male being sucked in to helping look after its brats can get left holding the baby. With mammals it is very rarely if ever possible because of the huge initial imbalance in investment - not only do females produce the egg, but they gestate and suckle the young as well.
Male mammals have it easy.
In those situations where the male is the sole carer, it is now he, rather than the female, who has the most to offer a prospective romantic interlude. And it will thus be she who must compete for his compliance. With polyandry the selective pressures for larger females are even stronger. Most animals show a strong aggressive tendency to other conspecifics, which is overcome only during mating seasons. My hunch is that the ability to hunt together requires a fair amount of brain size, which only began to appear with carnivorous mammals.