Hoarding by seed eating rodents often contributes to seed dispersion of several tree species bearing large seeds such as walnuts and acorns. However, it may be generally difficult for small rodents to eat a large hard-walled seed and transport it away for later usage. In central Japan, large (40 g) Apodemus speciosus and small (15 g) A. argenteus mice are distributed sympatrically and eat possibly the Japanese walnut seeds (Juglans ailanthifolia), including a variant J. ailantifolia var. cordiformis which produces much smaller seeds. To examine the size effects of seed/mouse on feeding and hoarding, first, we compared behavior of two mice when given with large (8.4 g) and small (4.7 g) walnuts under the laboratory conditions. Mice ate the both types of walnuts similarly in A. speciosus and also in A. argenteus, but the feeding rate was much lower in the latter mouse. Next, we compared the removal rates of large and small walnuts from the feeding boxes set in the field. A. speciosus removed walnuts randomly, whereas A. argenteus removed small nuts more frequently. These results suggest that a smaller species can eat walnuts in some degrees but prefers smaller nuts when transporting to hoard nuts probably because of higher transporting costs of the larger nuts.