Across species, body size plays a fundamental role in reproductive success, survival, and population resilience. However, attaining large size requires an increased investment into growth. My research aims to explore the balance between investment into growth and improved fitness mediated by reproductive success using a combination of theoretical modeling and empirical data collection. Through my research, I documented the cost of growth in marine mammal species to provide better estimates of investment into growth. Additionally, I used Stochastic Dynamic Programming (SDP) to quantify reproductive success across three size classes of harbor porpoise (Phocoena phocoena). Harbor porpoises are one of the smallest marine mammal species, making them an ideal candidate to explore the limitations placed on body size and benefits associated with attaining large size. Finally, I aim to use Bayesian hierarchical modeling to document which metrics of body size most accurately predict reproductive success in marine mammals. Using long-term data collected on northern elephant seals (Mirounga angustirostris), I will explore the components of body size that result in high reproductive success across ages. This research will aid in our understanding of the constraints placed on body size and benefits associated with attaining large size, while providing a framework to use body size as a proxy for individual and population health.