Life history theory attempts to define the pattern of resource distribution among offspring and predicts that egg size should be positively correlated with offspring fitness. In this paper we investigated the effect of an array of ecological and reproductive factors on the size of eggs laid by Lobesia botrana, and the ecological significance of egg size by experimentally testing whether or not egg size increased larval performance in this moth. Egg size was significantly affected by female age at mating, water availability, pupal weight loss, the phenological stage of the vine in which larvae developed, female body weight, and oviposition day, but was unaffected by the size of the spermatophore received. The greater the size of the first eggs produced, the smaller the size of the last eggs laid, as predicted by the resource depletion hypothesis. As expected, the larger the size of eggs the larger the size of larvae emerging from them. Large larvae displayed a better ability to endure starvation than small ones. When small and large larvae were allowed to develop on grape clusters, an adverse environment, large larvae performed better, settling and survival being significantly enhanced, almost tripled. Instead, when the same experiment was carried out on an artificial diet in the laboratory, a much more favourable milieu, survival in both sized-larval groups was not significantly different. We discuss our results in the context of current ideas relating egg size, larval performance and fitness in the Lepidoptera.