Growth within plant aerial lateral organs, such as leaves and flowers, occurs for a given period of time and stops when the organs reach their final size and shape, which are highly reproducible for genetically identical organisms grown under equivalent environments. Although leaf size is at one level simply a function of cell number and size, accumulating evidence suggests that an organ-wide integration system underlies leaf organogenesis and controls leaf size. This system, which integrates cell proliferation and expansion, has been a topic of vigorous research in recent years. In our research, we have focused on the intriguing phenomenon known as "compensation." In mutants that have a severe decrease in cell number, excessive cell enlargement is induced post-mitotically; therefore, compensation itself underscores the link between cell number and size-control systems at the level of the entire organ. In a recent large-scale histologic study of mutants of the model plant Arabidopsis thaliana, we identified a large number of leaf-size mutants with various combinations of cell-number and cell-size phenotypes. Five of these mutants, fugu1-fugu5, exhibited compensation phenotypes. Here, we highlight the recent advances in our understanding of size control and the possible mechanisms of compensated cell enlargement based on our analysis of compensation-exhibiting mutants.