Studying urban forms and dynamics is crucial for achieving urban sustainability. However, the influence of different urban forms on city expansion and suburbanization remains largely unknown. This study introduces an Urban Form Index (UFI) based on urban land density and applies it to 151 metropolises in Africa, China, and India (ACI) to assess the urban forms and their effects on expansion. The UFI categorizes cities into four types: compact, highly compact, sprawled, and highly sprawled. African and Indian metropolises are mainly compact/highly compact, while China leans toward sprawled forms. Analyzing the UFI values from 1985 to 2015 reveals 16 dynamic urban forms. African and Indian cities maintain or transition toward compact forms, while Chinese cities shift from compact to sprawled forms. Urban expansion rate, land density distribution, spatial extent, and built-up density are influenced by these forms, with regional variations. Recognizing diverse urban expansion trends in the Global South questions universal planning theories and emphasizes the need for context-specific urban planning to ensure sustainable urbanization in the future.