We studied ontogenetic shifts in sexual dimorphism and female reproduction in the Reeves's Butterfly Lizard, Leiolepis reevesii (Agamidae), from Hainan, southern China. The smallest reproductive female was 84 mm snout–vent length (SVL). The largest female and male were 136 mm and 166 mm SVL, respectively. Sexual dimorphism in head size and tail size was evident in adults, with adult males having larger heads and longer tails than did adult females of the same SVL. Head size relative to SVL was greatest in hatchlings and smallest in adults, with juveniles in between. Tail length relative to SVL was shorter in hatchlings than in juveniles and adults but did not differ between juveniles and adults. Females produced a single clutch of 2–8 eggs per breeding season stretching from mid-April to mid-July. Clutch size, egg mass, and clutch mass were all positively related to female SVL. Egg mass was negatively correlated with clutch size when holding female SVL constant, and egg length was negatively correlated with egg width when holding egg mass constant. Larger females increased reproductive output primarily through production of more and larger eggs (and hence heavier clutches). Males are the larger sex in L. reevesii presumably because (1) the selective pressure toward increased male size is strong; (2) the selective pressure toward increased female size is comparatively low; and (3) somatic growth is more likely to be constrained by reproduction in smaller females because of a negative correlation between relative clutch mass and SVL.