Democratic governments have constantly added new policies to existing policy stocks to confront societal, economic, and environmental challenges. This development has the potential to overburden public administrations in charge of policy implementation. To address this issue, we theorize and analyze how the relationship between the size of sectoral policy portfolios and implementation capacities affects sectoral policy performance. Our Bayesian analysis of the environmental policies of 21 Organisation for Economic Co-operation and Development countries from 1976 to 2020 reveals a widening “gap” between the policies up for implementation and the implementation capacities available and shows that this gap negatively affects environmental policy performance. Qualitative insights from 47 in-depth interviews with implementers validate these findings and shed light on the underlying causal processes. Our findings suggest that in advanced democracies transforming additional policies into effective problem-solving crucially hinges on the deliberate expansion of implementation capacities.