Why are the main urban areas of Foshan and Dongguan in Guangdong small?

Looking at Foshan, the main city of Foshan is really small, and I feel that I can't find the model of a big city and an economic center. This is mainly because the Buddha came to see Dongguan again. Wanjiang District, Chengnan District, Guancheng District and Dongcheng District in Dongguan are not in the usual sense. Dongguan, a prefecture-level city, is very special and directly governs the town. Because of Dongguan's special history and developed town-level economy, these towns and non-traditional areas are equivalent to county-level in Dongguan system. At that time, the township economy and county economy were relatively developed, while Foshan was formed by the integration of Shunde, Nanhai, Sanshui, Gaoming and other counties and cities (Chancheng and surrounding main urban areas).