In recent years, the compact city strategy has gained attention in South Korea as a solution to social issues from population decline and aging in regional cities. This study explores the application of the compact city strategy in declining population areas by measuring urban compactness based on population distribution patterns in the Busan-Ulsan-Gyeongnam metropolitan area, using data for the years 2015 and 2020. Compactness was assessed through four indicators: density, clustering, equal distribution, and dispersiveness. The analysis was conducted at both the neighborhood and city-wide levels. Findings showed a positive correlation between population change and compactness, suggesting that regions with declining populations might experience further compactness reduction. Compactness increased at the neighborhood level due to the redevelopment of single-family homes into multi-family apartments, but decreased at the city-wide level, likely due to suburban development. This study highlights the need to evaluate compactness at different scales and suggests prioritizing compact city strategies in areas experiencing population decline, with adjustments at the city-wide level.