Many ectotherms grow more slowly but mature at a larger size in colder environments, according to the pattern called the temperature-size rule. Thermal variation of cell size is suspected to be inherent in the origin of this pattern, but empirical testing of this hypothesis has been conducted in only a few taxa. In the laboratory, we reared two subspecies of the land snail Cornu aspersum (formerly Helix aspersa), C. a. aspersum and C. a. maximum, at two thermal regimes (15 and 20 °C), aiming to examine the relationship between cell size and adult mass across temperatures. The warmer environment led to larger adult mass in both taxa, contrary to the temperature-size rule. Using histological techniques, we found that snails grown at the warmer temperature produced heavier shells, smaller muscle and epithelial cells, but larger hepatopancreatic cells and nuclei. These results strongly suggest that the temperature-size rule for body size cannot be explained by a simple consequence of thermal sensitivity of cell size. Focusing on the patterns between the two taxa, we found that aspersum grew more slowly, with faster metabolic rates and smaller mass at the end of the experiment, when compared with maximum, regardless of temperature. The subspecies aspersum evolved larger muscular and epithelial cells than maximum, but both had a similar size of hepatopancreatic cells and a similar shell mass. Interestingly, when compared across a range of small body masses, aspersum had a heavier hepatopancreas than maximum. We propose that the evolutionary divergence between the two subspecies might involve a trade-off between growth and hepatopancreatic functions, but not a trade-off caused by the costs of shell production and maintenance of cell membranes.