Data sources.

Urban agglomerations, such as the Yangtze River Delta, Yangtze River Middle Reaches, and Chengdu-Chongqing regions, play a crucial role in driving China’s regional economic development. While previous studies have focused on economic and social aspects, the fiscal dimension of urban agglomerations remains underexplored. This study addresses this gap by investigating the relationship between population size and fiscal efficiency in these three major urban agglomerations along the Yangtze River Economic Belt (YREB).We introduce the concept of fiscal efficiency based on revenue and expenditure and select relevant indices, such as efficient population size and fiscal self-reliance. Using statistical data from 2017 to 2019, we employ curve regression analysis in SPSS to estimate the efficient population sizes of these urban agglomerations and examine differences in financial efficiency over time and space. Our analysis reveals that cities with populations over 10 million hinder fiscal efficiency in the Yangtze River Delta, those with 3–5 and 5–10 million in the Yangtze River Middle Reaches, and those with 5–10 and 1–5 million in the Chengdu-Chongqing urban agglomerations. The maximum financially efficient population sizes are estimated at 648 million for the Yangtze River Delta, 308 million for the Yangtze River Middle Reaches, and 320 million for the Chengdu-Chongqing urban agglomerations. Considering various fiscal indicators, all three agglomerations demonstrate varying degrees of efficiency. The innovation of this study lies in the interdisciplinary approach, integrating finance, demography, urban planning, and regional economics. By analyzing population size from a fiscal perspective, we provide a novel theoretical framework and analytical tool for policymakers. This study highlights the importance of fiscal balance and population optimization in urban agglomerations, contributing to regional coordinated development and sustainable growth.