Agricultural eco-efficiency is a crucial indicator for measuring the green and sustainable development of agriculture. This study utilizes provincial panel data from 2006 to 2019 to measure agricultural eco-efficiency while integrating carbon emissions and carbon sinks. A two-way fixed effect model and a nonlinear quadratic term model are utilized to investigate whether expanding farmland management scale contributes to enhancing agricultural eco-efficiency. The research results indicate that agricultural eco-efficiency is at a moderate level and exhibits a positive trend with an average annual growth rate of 1.28%. The relationship between farmland management scale and agricultural eco-efficiency is characterized by an inverted U-shaped nonlinear association. Appropriate management scale is essential for promoting green development of agriculture and is validated even after conducting endogenous and robustness tests. The research identified a significant moderating effect of service outsourcing, where a higher degree of service outsourcing results in a flatter inverted U-shaped curve mentioned above. Moreover, the non-linear relationship between the scale of agricultural land and agricultural eco-efficiency has become more significant after the reform of “separation of the three rights” and in the east and central regions. Finally, based on the above conclusions, the paper provides specific recommendations for improving agricultural eco-efficiency.