AbstractPersonal, work, and societal concerns about obesity and body fatness have triggered research on it across multiple domains. However, the organizational literature has been hampered by a significant shortcoming in considering it solely as an objective construct, despite research in other disciplines demonstrating a critical subjective component to how body fatness is experienced. To address this conceptual and theoretical challenge, we draw on stigma theory to explore the workplace implications of subjective fatness, or how big one feels in their own mind. We utilize and extend stigma theory by integrating it with medical research. In doing so, we shed new light on the subjective nature of the self‐devaluation process that occurs in stigmatized individuals. We argue that this self‐devaluation process is the mechanism by which subjective body fatness influences work performance. We test these hypotheses across three studies that constructively replicate results across multiple design types (multi‐rater, time‐lagged, & bivariate latent‐change model) and diverse geographic samples. Results consistently show that subjective fatness exerts a stronger impact on performance than objective fatness, and this influence is mediated by self‐devaluation. In all, our work indicates it is not how big one is but how big one feels that most affects work performance.