• Premise of the study: Specifi c leaf area (SLA) is a critical component of the leaf economics spectrum, and many functional leaf traits have been empirically demonstrated to covary with SLA. However, a complete understanding of how change in leaf size influences SLA has not yet emerged. • Methods: To help develop a more complete understanding of the determinants of variability in SLA, we present a covariation model of leaf allometry that predicts a zero-sum interdependence of leaf thickness, density, and surface area on leaf mass. We test the model ’ s predictions on measurements of 900 leaves from 44 angiosperm species. • Key results: We observe that “ diminishing returns, ” the negative allometry (slope < 1) of surface area versus mass, does not hold universally across species. Rather, the scaling of SLA is linked to the relative allocation to thickness and density. Specifically, diminishing returns are observed when leaves grow thicker, more than their density decreases, with increasing mass. Finally, we confirm model predictions that the allometric dependence of area, thickness, and density on mass can be well approximated by a zero-sum allocational process. • Conclusions: Our work adds to the growing body of evidence that allometric covariation is a hallmark of the scaling behavior of complex plant and leaf traits. Moreover, because our model makes predictions based on allocational constraints, it provides a foundation to understand how deviations from zero-sum tradeoffs in allocation to leaf thickness, density, or area determine the allometry of SLA and, ultimately, underlie adaptive strategies within and across plant species.