"developing" in this sense usually refers more to GDP per capita and other measures. China is the world's largest economy, but even in Purchasing Power Parity terms it's 75th (on a per capita basis), similar to places like Mexico and Thailand. In many measures a Chinese person living in Shanghai lives a life that is almost indistinguishable from say an American in New York City, but there are a lot of much poorer places in China that have a long way to go before they're "developed."
"developing" in this sense usually refers more to GDP per capita and other measures. China is the world's largest economy, but even in Purchasing Power Parity terms it's 75th (on a per capita basis), similar to places like Mexico and Thailand. In many measures a Chinese person living in Shanghai lives a life that is almost indistinguishable from say an American in New York City, but there are a lot of much poorer places in China that have a long way to go before they're "developed."