The size of seeds and organs is coordinately determined by cell proliferation and cell expansion, but the mechanisms that set final seed and organ size are largely unknown in plants. In a recent study, we have demonstrated that the plant specific G protein γ subunit (AGG3) promotes seed and organ growth by increasing the period of proliferative growth in Arabidopsis. AGG3 is localized in plasma membrane and interacts with the G protein β subunit (AGB1). Homologs of AGG3 in rice (GS3 and DEP1/qPE9-1) have been identified as important quantitative trait loci for seed size and yield. However, rice GS3 and DEP1 influence seed and organ growth by restricting cell proliferation. Here, we discuss the possible molecular mechanisms by which Arabidopsis AGG3 and its rice homologs GS3 and DEP1 control seed and organ size.