To explain the general tendency of large mothers to produce large offspring, we developed two models in which either the rate at which each single offspring extracts resources from the mother or the rate at which the mother supplies resources to all the offspring is limited (terminal- or upper-stream-limitation on resource transport, respectively). We also reanalyzed the data of Erythronium japonicum to test the models. The terminal-stream-limitation model predicted that the optimal offspring size that maximizes the fitness of the mother increases with an increase in the maximum rate of resource extraction by each single offspring. Thus, large mothers produce large offspring if the maximum resource extraction rate is high in those mothers. The upper-stream-limitation model predicted that the optimal offspring size decreases with an increase in the maximum rate of resource supply by the mother to all the offspring. In E. japonicum, the maximum growth rate of a seed was independent of the number of seeds of a plant, suggesting that the resource extraction rate is limited at the individual seed level. The maximum growth rate was high in large plants and had a strong positive effect on final seed mass. Thus, the results were consistent with the terminal-stream-limitation model.