Size inequality has been considered a key feature of plant population structure with impacts on ecosystem functions. In forest ecosystems, studies examining the relationship between tree size inequality and stand productivity have produced mixed outcomes. Most studies have discussed positive or negative relationships with regard to competition for light between trees (e.g. light interception efficiency), but far less attention has been paid to the role played by tree ontogenetic growth. In this article, we present a simple mathematical model that provides the ontogenetic basal area growth of trees in a stand having two strata and that includes size-asymmetric competition between trees. Comparing the growth of this stand to the growth of a reference stand with only one stratum, we show that higher growth of the two-strata stand is achieved for low intensities of absolute size-asymmetric competition and for concave shape, increasing functions of ontogenetic growth. We also demonstrate that the difference in growth between the two stands depends on tree density, stand quadratic mean diameter and tree size inequality in the two-strata stand. We highlight that different relationships between size inequality and productivity can be achieved (positive, negative and nonmonotonous). Overall, our results highlight that ontogenetic growth has a major impact on these relationships. We then discuss the consequences for future studies analysing the effect of size inequality on forest productivity. Our approach calls for new theoretical developments to deal with size inequality and ecosystem functions in plant populations.