Your “few bad cities” also contain 24.5/40.0 million (Shanghai), 22.5 million (Beijing), 19.8/44.3 million (Guangzhou), 15.5/16 million (Tianjin), 15.4/25.2 million (Chongqing), 14.7 and 18 million (Chengdu), 12.4 million (Shenzhen), 10.7/19.8 million (Wuhan) and 10.0/21.1 million (Hangzhou).¹ China is 58% urbanised.² Most of the land area of China is not as polluted as Shanghai, that is true; but disproportionately many people live in those areas—which is unsurprising, because pollution is often due to high population density. (Pollution per capita is of coursed generally reduced by density, but that is another matter.)

1. see the Wikipedia page for largest cities in China

2. Yiping XIAO, Yan SONG and Xiaodong WU, “How Far Has China’s Urbanization Gone?”, in: Sustainability 10.8 (Aug. 2018), p. 2593, DOI: 10.3390/su10082953