<p>Urban growth and shrinkage are two sides of the same coin in the process of urbanization, and exploring their differentiated land use transformations and eco-environment effects is of great significance for coordinating regional land resource development and improving eco-environment quality. Taking the Southwest Karst region of China as an example, this study identifies growing and shrinking cities from 2010 to 2020 based on county-level population changes. We then conduct a comparative analysis of the spatiotemporal differentiation characteristics of land use transformation and eco-environment quality in growing and shrinking cities from the perspective of production-living-ecological space. Furthermore, the study employs the Geographic Detector model to identify the factors influencing the spatial heterogeneity of eco-environment quality in growing and shrinking cities. The results are as follows: (1) In both growing and shrinking cities, land use transformation was primarily characterized by an increase in production and living land areas, while ecological land area decreased. Specifically, ecological land area decreased by 0.89% in growing cities and 0.73% in shrinking cities. (2) From 2010 to 2020, there was a 0.86% decrease in eco-environment quality in growing cities and a 0.38% decrease in shrinking cities, with the gap between them narrowing. (3) Natural factors and socio-economic factors are important determinants of the spatial differentiation of eco-environment quality, but there is a significant difference in the spatial determinants and indicators between growing and shrinking cities. The interactive effects of these two factors exert a more significant influence on the spatial impact of eco-environment quality. These findings provide theoretical underpinnings for the optimization of territorial space and eco-environment protection in the Southwest Karst region of China.</p>