One of the long-standing paradoxes in genomic evolution is the observation that much of the genome is composed of repetitive DNA which has been typically regarded as superfluous to the function of the genome in generating phenotypes. In this work, we used comparative phylogenetic approaches to investigate if the variations in genome sizes (GS) should be considered as adaptive or neutral processes by the comparison between GS and flower diameters (FD) of 50 Passiflora species, more specifically, within its two most species-rich subgenera, Passiflora and Decaloba. For this, we have constructed a phylogenetic tree of these species, estimated GS and FD of them, inferred the tempo and mode of evolution of these traits and their correlations, using both current and phylogenetically independent contrasted values. We found significant correlations among the traits, when considering the complete set of data or only the subgenus Passiflora, whereas no correlations were observed within Decaloba. Herein, we present convincing evidence of adaptive evolution of GS, as well as clues that this pattern is limited by a minimum genome size, which could reduce both the possibilities of changes in GS and the possibility of phenotypic responses to environment changes.