Selection favors larger body size in many taxa. In some species, larger individuals are more active and bolder than smaller conspecifics, but the nature of this relationship is unclear: Is body size a predictor or a consequence of behavior? Any effect of behavior on body size may be through its effect on competitive ability, suggesting that the presence of potential competitors could exaggerate or suppress this relationship. In this study, we tested whether an individual's activity rates early in life predict its future body size by measuring same-aged, sibling eastern fence lizards before (8 d) and after (8 wk) they significantly diverged in body size. We tested for an effect of conspecific presence (potential competition) on the relationship between early behavior and future body size by housing some lizards in sibling groups and others individually, using a split-clutch design. Our results reveal that individuals' activity rates do not significantly differ between 8 d and 8 wk of age. At 8 wk (but not 8 d) of age, more active siblings were also larger in both housing treatments; however, early activity rates did not predict body size later in development for either of these groups. Conversely, body size at hatching did predict size at 8 wk. Although variation in activity rates exists prior to divergence in body size, and activity and body size are correlated later in development, our results suggest that higher activity rates are unlikely to be driving body size divergence in this species. Instead, very small differences in body size at hatching appear to be compounded over time and drive much more exaggerated differences in later body size.