We collected data on maternal mass, clutch mass (reproductive effort), number of offspring, and mean offspring mass from 28 species of Lycosidae (wolf spiders) and five species of Pisauridae (nursery-web spiders) found in Mississippi, USA. Our primary goal was to test for a trade-off between offspring number and offspring size (mass) among wolf and nursery-web spiders, which are sister families. The regression of reproductive effort on maternal mass was highly significant and explained 94% of the variation in reproductive effort among species and 96% of the variation among genera. The slope of the regression line between maternal mass and total offspring mass was not significantly different from one, suggesting that spiders used a constant proportion of their total energy budget for reproduction regardless of size. Partial correlation and principal components analyses demonstrated a clear trade-off between offspring size and number. Species with large offspring (relative to adult size) produced fewer offspring than expected. Lycosids produced small numbers of large offspring relative to pisaurids, and smaller species of both families are more constrained in the evolution of the offspring size:number continuum than larger ones.