Recent observations of body size declines in animal populations have given rise to discussions of whether or not this is related to climate change-induced temperature increases, with which the body size changes would follow Bergmann's rule. Although the debate is ongoing, the limited thermal benefits of currently observed size reductions make it unlikely that temperature increase shapes a direct selection pressure. Food constraints during early-life development, which could be caused by mismatches between available resources and energetic demands, could cause smaller body sizes too. Here we investigate whether a decrease in body size, observed in a migratory shorebird, the red knot (Calidris canutus canutus) at their West-African nonbreeding grounds over two decades, is linked to developmental plasticity during chick growth in the High Arctic. To do so, we combined datasets from both the wintering and breeding grounds on body size measurements (during chick growth and in fully grown juveniles), food availability, and diet inferred from stable isotopes deposited in feathers grown as chicks. From 2003 to 2021, stable-isotope ratios revealed a decline in the dietary contribution of crane flies (Tipulidae, Diptera), the key food of growing chicks in the Arctic. On the breeding grounds, we observed that while the emergence of adult crane flies advanced along with earlier snowmelt dates, red knots did not adjust the timing of breeding, and this resulted in an increasing mismatch with the demands of growing chicks. As a result, chicks grew slower and, as observed on the wintering grounds, reached smaller final body sizes. Our results imply that increasing resource-demand mismatches may lead to body shrinkage via plasticity during development. In this study, the increasing mismatch was linked with climate warming; the presented causal chain may explain other recent examples of body size reductions as well.