Male insects with larger body size tend to mate more often than smaller males. The effects of body size on mating were studied in the ground-nesting solitary bee Colletes perforator L. (Hymenoptera: Colletidae), which usually formed clusters during mating. Clusters consisted of a female and two or more males competing to mate with the female. Mating males did not differ in average size compared with nonmating males. In mating clusters with small females, however, the mating male was smaller on average than a nonmating male. In mating clusters with large females, mating males were larger than nonmating males. Males were observed to mate more than once unlike females, which mated only once. Males observed to copulate more than once were larger on average than those copulating only once.