Global change is increasing the occurrence of anomalously high sea surface temperatures and episodic deoxygenation, motivating mechanistic predictions of size-structured growth responses in cultured and wild fish. Using a stochastic individual-based bioenergetics model combined with experimental measurements, we simulate growth trajectories in Atlantic salmon across two size cohorts (~400–1300 g and 2600–3500 g) exposed to prolonged elevated temperature and mild hypoxia. Size-structured feeding hierarchy, a social organisational feature common to salmonids that persists regardless of environmental conditions, had the largest single effect on simulated growth, yet its influence on growth variation was modulated by the severity of physiological stress imposed by temperature and hypoxia. The relative importance of feeding hierarchy, stress responses, and individual variation in thermal parameters shifted with body size and environmental context. Individual differences in metabolic costs appeared to buffer hierarchy effects under suboptimum conditions, while stress-induced reductions in feed intake reduced competitive asymmetry under optimum conditions, suggesting that the mechanisms structuring growth variation are themselves environmentally contingent. Because our simulations overestimated growth in the smaller cohort, model improvements are required before drivers can be attributed with confidence, but the pattern of interactions identified here has implications for how size-structured growth models represent environmental stress responses.