Geographic size variation in ectotherms often reflects complex interactions between environmental and evolutionary factors, which can differ among taxa. In this study, we investigated body size patterns in two sister subspecies of spur-thighed tortoises, Testudo graeca whitei and T. g. marokkensis, across their natural ranges in North Africa. These subspecies show distinct climatic niches, and we hypothesized that their morphology would differ in response to environmental variables, such as climate, Normalized Difference Vegetation Index (NDVI, a measure of vegetation cover), latitude, and elevation. We measured tortoises across Morocco and Algeria to explore growth patterns and adult body size in response to these environmental factors. Growth models revealed significant sexual size dimorphism in both subspecies. The humid subspecies T. g. marokkensis exhibited a slower growth rate but reached a larger asymptotic size compared with the semiarid T. g. whitei. When evaluating the relationship between body size and environmental factors, we found that T. g. whitei exhibited a negative correlation between body size and latitude, which contrasts with Bergmann's rule. By contrast, T. g. marokkensis followed the rule, with body size positively correlated with both latitude and elevation. Our findings suggest that latitude and elevation are encompassing critical factors influencing body size, reflecting the impact of climatic factors like aridity on tortoise morphology. We also found that body size patterns aligned with niche divergence, with larger individuals of both sexes occurring in regions with more favorable climatic conditions. Indirectly, these results suggest that larger body size in T. graeca may be associated with higher fitness in both males and females.