Fruit size is one important criterion of fruit external quality affecting consumer acceptance. The effects of seed number on fruit size in two fleshy fruits, grape and tomato, of different genotypes and grown under distinct carbohydrate availability levels were analyzed with a model. The two-parameter model described within-fruit resource competition and was able to well represent the commonly observed decrease in fresh weight per seed along with the increase in number of seeds, regardless of species, genotypes, and carbohydrate levels that were evaluated in this study. However, carbohydrate levels largely modified the correlation between seed number and fresh weight per fruit. In grape, lowering carbohydrate level led to a decrease in the parameter reflecting the potential fresh weight per seed, while in tomato, the response to carbohydrate was genotype-dependent and concerned more the parameter reflecting the competition level for resources. The values of the latter parameter suggested an under-compensating competition for resources in domesticated grape and tomato genotypes. Finally, co-localizations among quantitative trait loci for fruit fresh weight and model parameters indicated that plant susceptibility to competition could underlie the fruit fresh weight determination.