Plant genomes vary in size and complexity, fueled in part by processes of whole-genome duplication (WGD; polyploidy) and subsequent genome evolution. Despite repeated episodes of WGD throughout the evolutionary history of angiosperms in particular, the genomes are not uniformly large, and even plants with very small genomes carry the signatures of ancient duplication events. The processes governing the evolution of plant genomes following these ancient events are largely unknown. Here, we consider mechanisms of diploidization, evidence of genome reorganization in recently formed polyploid species, and macroevolutionary patterns of WGD in plant genomes and propose that the ongoing genomic changes observed in recent polyploids may illustrate the diploidization processes that result in ancient signatures of WGD over geological timescales.