Uneven-aged forests are usually characterized by high size inequality of trees within stands. This dimension of heterogeneity has often been left out of the diversity-productivity debate which has mainly focused on species diversity. Our study aimed to bring quantitative answers to what effects size inequality has on the basal area production of pure forest stands. We used plot data (40 000 plots) from the French national forest inventory and retained the ten most representative species (number of plots > 200): Norway spruce, silver fir, Scots pine, maritime pine, sessile oak, pedunculate oak, pubescent oak, Aleppo pine, European beech, European larch. Using the Gini concentration coefficient as an index of size inequality, we showed through a statistical model analysis that this factor significantly reduced the productivity for eight out of the ten species studied. The pattern of variation of the effect could not be related to functional characteristics or traits differences between species. These results first tend to indicate that asymmetric competition is not compensated by competition release of dominant trees and second that self-tolerance is not driven by functional traits. We provide perspectives on the mechanisms that could explain such a pattern.