Tree growth rates generally increase with individual size, but how nutrient enrichment alters the size–growth relationship remains poorly understood. In this study, we asked whether a decade of nitrogen (N) addition altered the size–growth relationship of trees, and explored the potential mechanisms underlying such changes. We found a nonlinear shift in the size–growth relationship along the N addition gradient. The near size–symmetric growth observed in control plots shifted to a pattern favoring smaller individuals under low–level N addition (20 kg N ha⁻¹ yr⁻¹). This occurred because small trees had higher leaf N resorption efficiency in controls, reflecting stronger N conservation and allowing a stronger growth response to moderate N inputs. With further increases in N addition (> 50 kg N ha⁻¹ yr⁻¹), the relationship reversed toward size symmetry. This reversal was associated with N-induced changes in nutrient stoichiometry, as smaller trees showed greater phosphorus and potassium resorption efficiencies, suggesting they experienced stronger secondary limitation by these nutrients and consequently lost their growth advantage relative to larger trees. Our findings provide novel insights into how N addition rate reshapes the size–growth relationship of trees, highlighting multiple nutrient limitation as a potential driver of stand development. These results suggest that size–dependent growth responses should be incorporated into forest management and projections of long–term biomass accumulation under increasing N deposition.