Geographical gradients of body size express climate-driven constraints on animals, but whether they exist and what cause them in ectotherms remains contentious. For amphibians, the water conservation hypothesis posits that larger bodies reduce evaporative water loss (EWL) along dehydrating gradients. To address this hypothesis mechanistically, we build upon well-established biophysical equations of water exchange in anurans to propose a state-transition model that predicts an increase of either body size or resistance to EWL as alternative specialization along dehydrating gradients. The model predicts that species whose water economy is more sensitive to variation in body size than to variation in resistance to EWL should increase in size with response to increasing potential evapotranspiration (PET). To evaluate the model predictions, we combine physiological measurements of resistance to EWL with geographic data of body size for four different anuran species. Only one species, Dendropsophus minutus, was predicted to exhibit a positive body size–PET relationship. Results were as predicted for all cases, with one species – Boana faber – showing a negative relationship. Based on a mathematical model verified empirically, we show that clines of body size among anurans depend on the current values of those traits and emerges as an advantage for water conservation. Our model offers a mechanistic and compelling explanation for the cause and the variation of gradients of body size in anurans.