<p>The current study investigated the link between body weight (measured via BMI) and exercise-related cognitions (fitness mindset) and behaviours (exercise frequency and effort), as well as the mediating role of three psychosocial factors: self-presentational concerns, negative social comparison, and social physique anxiety. Data were collected cross-sectionally via validated questionnaires from a large sample of undergraduate psychology students (<i>n</i> = 707, <i>M</i>_<i>age</i> = 22.28, <i>SD</i> = 7.72). The findings challenge the notion that body weight is an impediment to exercise participation; although BMI predicted worse psychosocial outcomes, the overall mediation model revealed that BMI predicted more exercise frequency and effort. Moreover, the psychosocial variables mediated these associations, but in different directions. BMI predicted lower fitness mindset and less exercise frequency and effort via heightened self-presentational concern, and BMI predicted less exercise effort via more negative social comparison. Conversely, BMI predicted more, rather than less, exercise frequency and effort via heightened social physique anxiety. Negative self-referent evaluations (i.e., regarding effective impression management and relative standing) were more likely to be demotivating via reduced confidence and belongingness, whereas the threat of external judgement (i.e., regarding negative physique-related evaluation) was more likely to be motivating perhaps via compensatory mechanisms. The findings further illuminate the importance of considering the interaction between personal characteristics and psychosocial context when understanding exercise as a health-promoting activity for people across the full spectrum of body weight. Future research should disentangle the contexts in which psychosocial factors are motivating or demotivating for certain individuals, perhaps via in-the-moment methodologies.</p>