Density-dependence theory cannot account for the widely observed pattern of proliferation and concentration in organizational populations. Although density dependence provides an explanation for initial proliferation, it cannot explain subsequent concentration because it does not allow some organizations to become dominant competitors. To address this basic limitation, I combine density dependence with three ecological models that permit size-based competitive asymmetries among a population's members and let the intensity of competitive processes vary over time. My analysis shows that Manhattan hotels of different sizes generate and experience different strengths of competition and that size-based competitive processes increase in strength over time, contributing to industry concentration. Separating empirically the effects of low-density conditions that occur early and late in a population's history also clarifies tests of density dependence in populations that have evolved beyond their peak density.