1. Females of Dawson's burrowing bees vary in body weight over a twofold range. Despite the potential for differences in body weight to affect several aspects of the competition among nesting females, no clear advantages were documented for larger females.2. Nesting females were not consistently larger than emerging females, nor was there a consistent relationship between body size and the weight of pollen and nectar carried to the nest on provisioning trips. At one nesting location, larger females did not produce larger pre-pupal offspring nor did they produce offspring at a faster rate than their smaller nesting companions.3. In addition, large body size was not associated with greater success in nest defence despite the fact that nesting females regularly encountered intruders in their burrows. Residency, not body size, determined the outcome of almost all contests for control of a nest burrow. The absence of a large body size effect here appears to stem from an intruder strategy designed to enable nest-searching females to acquire burrows that had been abandoned through death or dispersal of their original owners rather than securing existing nests through an aggressive takeover strategy.4. Thus, although large body size conveys significant fitness advantages to males, this attribute does not promote female success in either provisioning or defending their nests.