This work investigated the link between genetic and developmental controls of fruit size and composition. On two isogenic lines (CF12-C and CF14-L), differing by fruit weight and sugar content quantitative trait loci (QTLs) identified previously, basal and tip fruits were characterized at anthesis and at maturity through their growth, dry matter and sugar content, number and size of cells and nuclei DNA content. The influence of competition was assessed by removing either basal or tip ovaries at anthesis. On an intact inflorescence, CF12-C fruits grew less than CF14-L fruits, with 1.67 fewer cell layers and similar cell size, suggesting that genes controlling cell division may be responsible for this fruit size variation. Truss thinning masked the QTL effect on fruit size, mainly by reducing the difference in cell number between the two lines and by promoting cell expansion in tip fruits, so that fruit growth was similar at both positions and for both lines. Thus, in these lines, cell number exerts a control on final fruit size only when there is competition among fruits. Different responses of basal and tip fruits after flower removal suggested that this treatment induced changes in hormonal relationships within the truss. No fixed relationship between DNA endoreduplication and cell size was found, as while cell size and dry matter and sugar contents differed with tomato lines, fruit position and truss size, endoreduplication patterns were the same. CF12-C fruits had a higher dry matter (+0.3% of fresh weight) and carbohydrates (+8% of dry matter) content than CF14-L fruits. The percentage dry matter was independent of truss size but decreased slightly from basal to tip fruits.