This study examines the extent to which market size disparity across franchises—measured as the coefficient of variation of MSA populations—moderates the effectiveness of competitive balance interventions (CBIs) in Major League Baseball (MLB), the National Football League (NFL), the National Basketball Association (NBA), and the National Hockey League (NHL) from 1967 to 2023. Using two-way fixed effects models with multiple balance measures, we find that CBI effectiveness depends on the distribution of market sizes across league members. Jointly adopting a salary cap and floor improves competitive balance at low levels of market size disparity but is ineffective at high levels. Revenue sharing shows limited effects. Luxury taxes are associated with worsened competitive balance in high-disparity leagues. Our findings demonstrate that market size disparity not only affects competitive balance directly but also determines which interventions succeed. These results have direct relevance to recent discourse on competitive balance in MLB.