Both abiotic and biotic factors influence population and community structure. However, the simultaneous assessment of the relative importance of both types of factors is rarely performed for multiple traits of a population, such as body size and abundance. Comparisons among different demographic rates are necessary for teasing apart the importance of species interactions and/or environmental conditions on both population and community structure. We tested whether biotic (e.g., larval competition) or abiotic factors influenced larval salamander density and body size in natural populations of two known salamander competitors, Ambystoma talpoideum and A. maculatum. Over six years, we surveyed 33 ponds where these species co-occur in western Kentucky, USA. We found that larval densities between species were positively correlated, and that habitat features had contrasting, species-specific effects. Larval sizes for each species showed negative intra- and interspecific relationships with larval densities of each species, but larval A. maculatum generally exerted the stronger relative interspecific effect. Overall, our study highlights that different characteristics of a population (i.e., body size or abundance) may be differentially affected by abiotic and biotic factors, even for ecologically similar, sympatric species. Understanding which traits are regulated by each component will advance our knowledge on how populations and communities are structured.