<p>Metabolic scaling theory provides a framework to understand energy flows in ecosystems, predicting a negative relationship between body mass and density, where smaller species tend to occur in higher densities than larger ones. We assessed disproportionately high densities in tropical stream macroinvertebrates through size-density relationships (SDR), focusing on taxa exceeding expected densities for their body mass. We hypothesized that unique functional traits allow singular niche exploitation and reduced competition, resulting in high net energetic gains. To test this, we sampled Atlantic Forest stream communities monthly for over a year, estimating individual body mass via allometric equations. Positive SDR residuals indicated disproportionate densities, which we related to functional traits. As expected, size and density were negatively related, with some taxa showing greater density in time than predicted by their body mass. However, contrary to our hypothesis, functional uniqueness was not associated with higher densities, suggesting that traits alone may not drive disproportionate densities. Instead, disproportionate densities appeared in taxa with high redundancy with congenerics. Additionally, taxa with higher densities showed greater temporal variability, indicating a fluctuating dynamic of dense populations. Our results suggest that high densities could be linked to a combination of niche preemption and modification mechanisms of community assembly.</p>