Question

If having large numbers of offspring results in high fitness, why don't all females have very large numbers of offspring?

Answer 1

Females does not have large number of offspring

If having large numbers of offspring results in high fitness. All females have very large numbers of offspring because they are restricted by the value of energy they can dedicate to reproduction in any one season. Reproduction also depends upon amount of energy of an organism.

Answer 2

"They are limited by the amount of energy they can devote to reproduction in any one season." is the best answer.

Having children comes with a cost, lowering the female's survival skills due to time and energy spent in raising offspring. A trade-off between reproduction, longevity and parental investment exists.