Females vary in the size of offspring that they produce, often in a manner that depends on maternal age or stage. This is puzzling, given that offspring size is predicted to evolve to a single optimal value where the gain in fitness from being larger exactly offsets the fitness lost to the mother by producing fewer offspring. We used a stage-structured life-history model to determine the optimal offspring size for females in different stages. We found that optimal offspring size does not vary with maternal stage when offspring fitness depends only on its size and not on the stage of the mother. This negative result holds even with density dependence, when larger offspring compete better. However, a trade-off between offspring size and maternal survival affects the optimal offspring size. The future reproductive value of the female, coupled with the costs and benefits of offspring investment, drives the evolution of stage-dependent offspring size. If producing larger offspring is riskier for mothers, females produce smaller offspring when their reproductive value in the next time step is large relative to current reproductive prospects. These analyses provide a novel framework for understanding why offspring size varies in age- and stage-structured populations.