Objectives This study investigates allometric changes in shoulder breadths relative to changes in stature arising from rapidly changing developmental circumstances within 107 Taiwanese families. It speaks to broader issues related to the extent of phenotypic plasticity of body breadths humans are capable of in response to reductions in developmental stressors. Methods An examination of relationships between shoulder breadth and height within individuals in each generation was followed by evaluation of patterns of difference between same‐sex parent–offspring pairs in height and shoulder breadth. Results Height was similarly positively correlated with shoulder breadth within fathers, mothers, sons, and daughters (P ≤ 0.002). Variance accounted for ranged from an adjusted R2 of 0.201 among fathers to 0.151 for sons, with mothers' and daughters' values being 0.187 and 0.181, respectively. Comparisons of differences within families indicate that parents who were shorter than their same‐sex offspring also tended to have modestly narrower biacromial breadths (father–son pairs: adjusted R2 = 0.112; t = 2.82, P = .007; mother–daughter pairs: adjusted R2 = 0.135; t = 2.97, P = 0.005). Conclusions Taken as a whole, results here support the view that secular changes in stature are not accompanied by similar changes in body breadths, perhaps so that responses to developmental environmental improvements do not alter thermoregulatory equilibria that reflect long‐term evolutionary processes. These results indirectly constrain plausible hypotheses about how ancestors of Austronesian speakers altered their body size and shape as they voyaged to Fiji, Western Polynesia, and beyond.