Phenotypic variation in a population is a key requirement for natural selection. Body size variation, in particular, can affect the dynamics, life history, and ecological relationships in a population. Past work suggests that variation in body size within a population reflects a competitive gradient in which larger individuals negatively affect smaller individuals because of the superior ability to obtain limited resources, a result of exploitative or interference competition. We hypothesized that, if larger individuals out-compete smaller individuals through aggressive interactions (interference competition), the individual's position in the size distribution of their population (relative size) would be a better predictor of growth rate and time to metamorphosis than would their absolute size. We tested for the effect of body size on future larval growth rates, time to metamorphosis, and aggressive behaviors in individual Ambystoma talpoideum using experimental microcosms. Larvae were reared in small groups, producing variation in growth and development within each group. Using mixed-model analyses, we found that relative size was the best predictor of time to metamorphosis and aggressive behaviors and that growth rate of individuals within these populations was best explained by absolute or relative size depending on life stage. The largest individuals within a tank made the most aggressive attacks, had faster growth rates when young, and the quickest time to metamorphosis, regardless of their absolute size. This study found evidence for a complex relationship between population size structure, growth rate, and time to metamorphosis by tracking the effects of relative size on individuals within a population.