Fruit size is a key criterion of the external quality of the fruit and may affect consumer acceptance. We used a model to analyze the effects of seed number on fruit size in two fleshy fruits, grape and tomato, of different genotypes, grown in different carbohydrate availability conditions. The two-parameter model described within-fruit resource competition and accurately represented the commonly observed decrease in fresh weight per seed with increasing seed number, regardless of the species, genotype or carbohydrate level considered. However, carbohydrate levels strongly modified the correlation between seed number and fresh weight per fruit, resulting in an increase, no change, or a decrease in individual fruit weight with increasing seed number per fruit. In grape, lowering carbohydrate levels decreased the parameter reflecting the potential fresh weight of one-seeded fruit under a given resource availability, whereas the response to carbohydrate level was genotype-dependent in tomato and more strongly reflected the level of competition for resources. The values obtained for this parameter suggested that there was an undercompensating competition for resources in domesticated grape and tomato genotypes. Finally, colocalizations of quantitative trait loci for fruit fresh weight and model parameters indicated that plant susceptibility to competition might determine fruit fresh weight