Rensch's rule, a well-studied macroevolutionary pattern of sexual size dimorphism (SSD), predicts, in monophyletic taxa in which males are the larger sex, that as the body size of the species in the Glade increases, SSD increases as well. We tested Rensch's rule in the superfamily Majoidea, a diverse Glade of 'spider crabs.' Considering that having the widest range of data available is important when testing macroevolutionary hypotheses, we first explored SSD in the pygmy spider crab Petramithrax pygmaeus (Bell, 1836), one of the smallest spider crabs in the superfamily. Petramithrax pygmaeus exhibits an unusual pattern of sexual dimorphism, with females displaying a larger average carapace width than males, yet males attain a larger overall body size. This type of SSD is unusual for spider crabs, with most species displaying male-biased sexual size dimorphism for both mean and overall body size. Hierarchical cluster and discriminant analyses demonstrated the existence of two distinct ontogenetic phases in P. pygmaeus (juvenile/prepubertal and adult/postpubertal). We used our data on body size of P. pygmaeus along with previously published body-size data for 11 other species within the superfamily Majoidea to test for Rensch's rule. Using raw data, phylogenetic independent contrasts generated from maximum likelihood (ML) and Bayesian inference (BI) phylogenetic trees, and reduced major axis regression analyses, we determined that for the tested species in the Majoidea that the predictions of Rensch's rule were not upheld. Macroevoultionary patterns such as Rensch's rule are not well studied in marine invertebrate systems; looking at how these rules compare between vertebrate and invertebrate and terrestrial and aquatic animals will help us to understand the differences and factors that could influence size and growth in these systems.