Competitive ability in plants is defined traditionally by a ‘size advantage’ hypothesis – i.e. larger species are generally expected to be more successful under competition because of greater capacity for resource capture, and thus capacity to deny resources to neighbours (e.g. through shading). We therefore tested the prediction (for crowded herbaceous vegetation) that species with a larger maximum potential body size (dry mass) should: (1) have generally increased resident plant abundance (i.e. more rooted units), resulting from more successful recruitment of reproductive and/or clonal offspring; and (2) account for a relatively large proportion of the standing biomass within crowded neighbourhoods.