Body shape and body size are hugely important for the understanding of multiple ecological phenomena. In order to study and compare sizes across taxa and to understand the ecological significance of shape differences, there is a need for ways to ‘translate’ different size measurements to a common metric. Body mass is the most useful such common index for size across taxa. Based on a large (>900 species in 28 families) dataset of lizard and amphisbaenian weights, I generate equations to estimate weights from the common size index used in lizard morphometrics (snout–vent length). I then use a species-level phylogenetic hypothesis to examine the ecological factors that affect the variation in weight–length relationships. Legless and leg-reduced lizards are characterized by shallower allometric slopes, and thus long-bodied legless species are lighter than legged lizards of comparable length. Among legged species, the foraging strategy strongly influences the weights, with sit-and-wait species being bulkier at comparable lengths than active foraging species. Environmental productivity (positively related to mass) and activity times (diurnal species being heavier) are only significant when using non-phylogenetic models. The need for effective locomotion is a major factor affecting lizard shape. Previously used allometric equations are inaccurate.