Burying beetles (genus Nicrophorus) use small vertebrate carcasses for reproduction. Body size varies widely, both within and among species. Across species, body size is related to the size of the carcass used for reproduction, with larger species tending to use larger carcasses and smaller species using smaller ones. Further, body size is known to determine the outcome of competitive interactions for carcasses both within and among species. Currently, there is no comprehensive analysis of what drives these differences in size and how body size is distributed within the genus and across its geographic range. Pronotal width correlates with overall body size in burying beetles, so we used a large dataset of pronotal width measurements and geographical data to evaluate the relative importance of phylogeny, biogeography, and ecology in explaining body size variation in burying beetles. 1) Mean body size distribution among species is broad (4.15–10.97 mm pronotal width) and skewed, with more small and medium-bodied species than large species. 2) The ancestral body size of the genus was medium (≈ 6.67 mm), and gigantism (> 9 mm) evolved at least three times, and small body sizes (< 5 mm) evolved at least five times. 3) We found evidence of phylogenetic signal in the evolution of body size across the genus, although only one instance of sister species being both giants, and no instances of sister species being both small. 4) Body size is divergent between most sister species and more strongly so between sympatric sister species. 5) Species richness and body size distribution covaries—areas with higher species richness have a greater spread between the largest and smallest species. 6) Smallest body sizes and highest species richness are concentrated in northern hemisphere temperate latitudes. Body size variation in *Nicrophorus *is driven in part by competitive interactions over carcasses, leading to divergence in resource use and corresponding beetle body size. High species richness and corresponding wide variation in body size in *Nicrophorus *assemblages only occur in temperate latitudes of the northern hemisphere. At a large geographic scale, character displacement according to body size appears to be an important factor in the coexistence of burying beetle species; however, the effects of other factors such as phylogeny, predation and parasitism, competition from non-nicrophorine species, and abiotic factors such as soil characteristics likely contribute to this divergence as well.

For each of these assessments, we used a dataset compiled by Derek S Sikes that included 12,019 pronotal width measurements from museum specimens and 5,247 pronotal widths from the scientific literature for 70 Nicrophorus species (See Sikes & Venables, 2013b for a list of museums from which specimens were borrowed). Each occurrence was georeferenced with corresponding latitude and longitude. Occurrence records from the literature, which lacked pronotal width data, were given mean values of the sex of their species, if known, or of their species, if sex was unknown. The full data set (museum + literature) was used to fill in gaps for the co-occurring species analyses, while only the museum data were used for the other analyses.