Larger sized butterflies can lay more eggs than smaller ones. In one study*, the size of wild collected cabbage white butterflies varied 3X, from 80 mg to 240 mg. Since smaller butterflies lay fewer eggs, why does the size of these butterflies not increase over time?
There are several competing factors in the equation. Not all variation in size is genetic. Some is nutritional. Caterpillars that develop on less nutritious resources or exhaust their food supply may pupate at a smaller weight which ensures some offspring (instead of zero). Smaller individuals may be able to exploit resources of marginal size that are too small to support larger individuals. Smaller individuals may develop more rapidly and in some conditions, have an additional generation. If there is a 20% difference in fecundity between the largest and smallest individual, that 20% could be greatly exceeded by an additional generation.
*N. Gilbert. Control of Fecundity in Pieris rapae: I. The Problem. Journal of Animal Ecology, Vol. 53, No. 2 (Jun., 1984), pp. 581-588.