It is well-known that whole genome duplication (WGD) has played a significant role in the evolution of plants. The best-known phenotypic effect of WGD is the gigas effect, or the enlargement of polyploid plant traits. WGD is often linked with increased weediness, which could be a result of fitness advantages conferred by the gigas effect. As a result, the gigas effect could potentially explain polyploid persistence and abundance. We test whether a gigas effect is present in the polyploid-rich geophyte Oxalis, at both organ and cellular scales. Methods: We measured traits in conspecific diploid and polyploid accessions of 24 species across the genus. In addition, we measured the same and additional traits in 20 populations of the weedy and highly ploidy-variable species Oxalis purpurea L., including measures of clonality and selfing as a proxy for weediness. Ploidy level was determined using flow cytometry. Results: We found substantial variation and no consistent ploidy-related size difference, both between and within species, and across traits. Oxalis purpurea polyploids did, however, produce significantly more underground biomass and more bulbils than diploids, consistent with a potential role of WGD in the weediness of this species. Conclusions: Our results suggest a more nuanced role for the gigas effect, at least in Oxalis. It may be temporary, short-lived, and inconsistently expressed and retained on evolutionary time scales, but in the short term can contribute to lineage success via increased vegetative reproduction.