<p>Urbanization drives land-use changes that contribute to rising temperatures and habitat fragmentation—factors that have induced phenotypic trait changes across many species. Body size is an important life history trait that can change with urbanization either through phenotypic plasticity or evolutionary change. Few studies distinguish the mechanisms behind trait change in urban environments, and even then, not many studies have focused on morphological traits. While body size of ectotherms may be expected to decrease with urban heat, habitat fragmentation could cause urban populations to increase in size, especially in taxa with a positive relationship between body size and dispersal ability, such as butterflies. In this study, we examined how a suite of traits related to body size (wing size, body mass, body segment mass), dispersal capacity (wing loading), and developmental stress (wing fluctuating asymmetry) vary along an urbanization gradient in the cabbage white butterfly (<i>Pieris rapae</i>). We used common garden experiments to distinguish between phenotypic plasticity and evolutionary responses. We show that phenotypic plasticity influenced traits in a manner consistent with the temperature-size rule: at higher developmental temperatures butterflies had decreased body mass and smaller wings. However, we found no evidence suggesting that evolutionary changes have led to increased size traits in urban butterflies in response to habitat fragmentation. Instead, we found that body mass plasticity increased only in suburban butterflies, which was driven by changes to abdominal mass plasticity. These findings suggest that plasticity, rather than evolutionary change, currently plays a more prominent role in shaping size-related traits in urban populations of <i>P. rapae</i>.</p>