Invasive species must colonize new habitats away from their native range; therefore, factors affecting plant dispersal play a key role in invasion. The ploidy level and genome size (or Cx value) can affect the dispersal traits, physiology, and ecology of invasive species over a few generations, generating individuals that can face fluctuating environments, exploit new ones, and compete with native species. Several studies have demonstrated that invasive species tend to have smaller genomes than their noninvasive congeners, which is explained by the role that the Cx value plays in phenotypic evolution and ecological tolerance. In order to test this hypothesis, we compare the genome size variation in Argentine populations (invasive range) vs. South African populations (native range) of S. madagascariensis. To meet our goals, we estimated the Cx value of invasive populations collected on field trips, while for native populations we considered available published data. We extracted the bioclimatic variables in order to establish the ecological amplitude in which the genome sizes may be distributed. Our results evidenced larger genomes in the invasive range than in the native one. Furthermore, we propose that large genomes of the invasive populations could be mainly explained by the founder genotypes effect and the anthropogenic introduction of this species to Argentina. In addition, we demonstrated that genotypes with big genomes can tolerate different environmental conditions from those of their native range. Therefore, they could present a greater ability for colonizing new environments. The implications and importance of ploidy level in the invasion of S. madagascariensis are discussed.