Several studies found a significant impact of city size on climate policies. Larger-population cities are likely to have a higher capacity to commit to and adopt climate protection programs. This study seeks to explore if there is a difference in vulnerability to climate change and adaptation responses between big and small cities in developing countries. The study analyzes the climate adaptation plans of 45 coastal cities in developing countries and uses ANOVA and multiple regression analysis with city size as a moderator. The study finds that city size influences divergently on the vulnerability level and adaptation response. While the vulnerability level is not significantly different between groups of city, the capacity for adaptation is notably different. This finding illustrates that in small cities, the vulnerability-adaptation relationship reflects the problem-solution association while in big cities, this relationship is based on the availability of resources and capacities. These findings recommend the focus areas associated with city size in climate change adaptation and a proportional resource distribution and assistance to small and big cities based on the adaptation needs that are determined by vulnerability level.