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Question Experimental evidence suggests that competition among plant species is generally hierarchical and that relatively large species are at a competitive advantage when competition is predominantly above-ground. However, regional species pools are dominated numerically by relatively small plant species, and small species generally have higher local densities of resident plants within natural communities. One explanation is that larger plant species suffer disproportionately more under effects of intra-specific competition (i.e. greater density dependence). We tested this prediction using ten herbaceous plant species in a competition experiment. Location Kingston, Ontario, Canada, glasshouse. Methods Using a glasshouse experiment, we tested whether relatively large species suffer disproportionately more in monoculture relative to mixtures of all ten herbaceous plant species. We measured the effects of competition on biomass production and survival by monitoring both in monocultures and mixtures of our species. Results Larger plant species suffered more under intra-specific relative to diffuse inter-specific competition in terms of survival; however, the slope of this relationship was not significantly greater than one, indicating that larger species did not suffer disproportionate density-dependent suppression. Conclusions Our results support a role for size in plant competition, but also indicate that this role is reduced because relatively larger species suffer greater density-dependent mortality when competing with other, equally large plants. As such, size-based competitive hierarchies may not function as clearly in natural systems because the increased negative density dependence for larger species contributes to balancing out competition across size hierarchies. [ABSTRACT FROM AUTHOR]