Male Callosobruchus chinensis (Coleoptera: Bruchidae) have elaborated, pectinate antennae, which are absent from conspecific females and both sexes of a congener, Callosobruchus maculatus. To begin to unravel the mechanisms producing this striking dimorphism, we examined which morphological traits best explain body size variation in bruchid beetles and quantified sexual dimorphism of antenna size through allometric analyses. Using principal component analyses, we found that elytron length and pronotum width were significantly correlated with the first principal component, which was interpreted as explaining variation in body size. Regressions of log-transformed body size measures on log-transformed antenna length revealed that males of both species had longer antennae than conspecific females for any given body size, although most of this effect was attributable to higher intercepts, rather than increased allometry, in males. Comparisons among heterospecific males revealed that C. maculatus males have noticeably longer antennae than C. chinensis males at large body sizes. Callosobruchus chinensis males, thus, appear to have increased the receptive area of their antennae by adding to the width of, rather than further elongating, their antennae. Finally, we found evidence for positive allometry between log-transformed antenna length and log-transformed antenna width in C. chinensis males. We discuss our results in the context of evidence supporting the presence of an additional, and potentially unique, sex pheromone in C. chinensis females.