Understanding scaling relationships between twig size and leaf size along environmental gradients is important for revealing strategies of plant biomass allocation with changing environmental constraints. However, it remains poorly understood how variations in the slope and y-intercept in the twig–leaf size relationship partition among individual, population and species levels across communities. Here, we determined the scaling relationships between twig cross-sectional area (twig size) and total leaf area per twig (leaf size) among individual, population and species levels along a soil moisture gradient in subtropical forests in eastern China. Twig and leaf tissues from 95 woody plant species were collected from three sites that form a soil moisture gradient: a wet site (W), a mesophytic site (M), and a dry site (D). The variance in scaling slope and y-intercept was partitioned among individual, population and species levels using a nested ANOVA. In addition, the change in the twig–leaf size relationship over the soil moisture gradient was determined for each of overlapping and turnover species. Twig size was positively related to leaf size across the three levels, with the variance partitioned at the individual level in scaling slope and y-intercept being 98 and 90%, respectively. Along the soil moisture gradient, the twig–leaf size relationship differed inter- and intraspecifically. At the species and population levels, there were homogeneous slopes but the y-intercept was W > M = D. In contrast, at the individual level, the regression slopes were heterogeneous among the three sites. More remarkably, the twig–leaf size relationships changed from negative allometry for overlapping species to isometry for turnover species. This study provides strong evidence for the twig–leaf size relationship to be intraspecific, particularly at the individual level. Our findings suggest that whether or not species have overlapping habitats is crucial for shaping the deployment pattern between twigs and leaves.