In heterogeneous forests, large trees retain a substantial amount of above ground biomass, but their annual contribution to biomass accumulation remains unclear. A modal relationship between tree growth and tree size is traditionally expected. But recently, it has been demonstrated that the rate of tree biomass accumulation continuously increases with tree size supporting the metabolic theory of ecology. To clarify the role of large trees in biomass production of heterogeneous forest, we used data of tree growth, mortality and recruitment monitored during 20 years in 10×4-ha plots in a species rich and structurally complex tropical forest (Central African Republic). Biomass gains and losses were analyzed in relation to the abundance of large trees and by tree size classes using a bootstrap procedure. At the plot level, the accumulation of biomass in large trees was generally lower than that accumulated in small trees. The high mass growth rate of few large trees was therefore outbalanced by the growth of the numerous small trees. Moreover, the loss of biomass due to the mortality of few large trees could be substantial, and rarely outbalanced by the accumulation of biomass in these large trees. The annual net accumulation of biomass significantly decreases with the initial abundance of large trees.