It has long been known that organismal complexity is poorly correlated with genome size and that tremendous variation in DNA content exists within many groups of organisms. This diversity has generated considerable interest in: (1) the identity and relative impact of sequences responsible for genome size variation, and (2) the suite of internal mechanisms and external evolutionary forces that collectively are responsible for the observed diversity. Genome size in any given taxon reflects the net effects of multiple mechanisms of DNA expansion and contraction, which by virtue of their complexity and temporal juxtaposition, may be challenging to tease apart into their constituent contributions. Here we review our current understanding of genome size variation in plants and the spectrum of mechanisms thought to be responsible for this variation. We present a synopsis of the insights into the mechanisms and pace of genome size change that are uniquely facilitated by a phylogenetic perspective, particularly among closely related species. We also highlight recent studies in diverse angiosperm groups where comparative genomic approaches have yielded general insights into the myriad mechanisms responsible for much of the observed genome size variation, most prominently the contribution of transposable elements (TEs). Finally, we draw attention to the possibility of divergence in the relative importance of different mechanisms of genome size evolution during cladogenesis.