Growth Synergy versus Environmental Trade-Offs: Exploring Coordination Mechanisms among Polycentricity, Urban Development, and Carbon Emission Efficiency in Chinese Cities

China’s urban development is facing a critical paradox amid intensifying global climate imperatives. As rapidly expanding economies are exacerbating resource–environment trade-offs, shrinking cities are experiencing rising emissions owing to infrastructure redundancy and carbon lock-in effects. This dualistic trajectory challenges national decarbonization goals, thus necessitating spatial strategies to align urban restructuring with sustainability. Leveraging a coupling coordination degree (CCD) framework—integrating urban comprehensive development, polycentric spatial equilibrium, and carbon emission efficiency—this study analyzes 281 Chinese cities from 2012 to 2021. The results reveal divergent pathways: expanding cities demonstrate gradual CCD improvements, while shrinking cities face persistent coordination instability and systemic fragility. Random forest analyses identify policy intervention and technological innovation as key drivers of CCD improvement, but excessive governance risks undermining market resilience. Critically, this study challenges the universal applicability of polycentric solutions and exposes systemic vulnerabilities in shrinking contexts—issues that were overlooked in conventional urban sustainability paradigms. These findings support adaptive governance frameworks that prioritize regional diversity, balanced market-policy synergies, and long-term investments in green innovation to unlock low-carbon transitions in resource-dependent cities.