Testing two competing hypotheses based on different theories (i.e., the theory of career mobility and social exchange theory), this study examines whether organizational size enhances or buffers the negative relationship between perceived overqualification (i.e., having more education or qualifications, skills, and/or experience than one's job requires) and work engagement (a positive, affective-motivational state of fulfillment characterized by vigor, dedication, and absorption). The study sample consisted of 107 university graduates. A time-lagged design with two data collection points was implemented. Data were analyzed using moderated regression. The results showed that organizational size enhanced the negative relationship between perceived overqualification and engagement (vigor, dedication, and absorption). This is the first study to show the moderator role of organizational size in the investigated relationship. To mitigate the dysfunctional consequences of perceived overqualification on work engagement, we suggest that large organizations should give overqualified employees the opportunity to develop long-term, high-quality relationships in their work environment.