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1. In burying beetles (Nicrophorinae), body size is known to provide both a fecundity advantage (in females) and successful resource defence (in males and females). Despite this, considerable variation in body sizes is observed in natural populations.

2. A possible explanation for the maintenance of this variation, even with intra- and inter-specific resource competition, is that individuals might assort according to body size on different-sized breeding resources.

3. We tested prediction that ’bigger is always better’, in the wild and in the laboratory, by experimentally manipulating combinations of available breeding-resource size (mouse carcasses) and competitor’s body size in Nicrophorus vespilloides (Herbst 1783).

4. In the field, large female beetles deserted small carcasses, without breeding, more often than they did larger carcasses, but small females used carcasses indiscriminately with respect to size. In the laboratory, large beetles reared larger broods (with more offspring) on larger carcasses than small beetles, but on small carcasses small beetles had a reproductive advantage over large ones. Offspring size covaried with carcass size independently of parental body size.

5. The present combined results suggest breeding resource value depends on an individual’s body size, and variation in body size is environmentally induced: maintained by differences in available carcass sizes. This produces amechanism by which individual specialisation leads to an increase in niche variation via body size in these beetles.