Selfed and auxin-treated fruits of tomato (Lycopersicon esculentum Mill.) were obtained from tetraploid and diploid plants and compared to determine why selfed tetraploid fruits are smaller. Application of auxin to ovaries produced tetraploid fruits as large as normally set (selfed) diploid fruits but led to reduced fruit size in diploid fruits, reflecting a further potential for tetraploid fruit development. The selfed tetraploid fruits were found to be smaller because they had fewer cells per fruit. Cell size (diameter) at the end of fruit development was similar for all fruit types. DNA per cell did not closely match cell size during development, especially during the cell division stage. Protein per cell also failed to follow cell size. Protein per DNA showed one peak at around day 10 of fruit development and possibly a second one at day 30.