Population numbers turn out to be pretty important for rendering places at the correct zoom level. Since ‘town’ or ‘city’ is the highest possible qualification for any large urban centre (whether a sleepy 100 000 population suburb or a 10 million population metropolis) there may be many of these places very near each other. On lower zoom level, the renderer has to make a choice on which cities to show. Without any population numbers, no decision can be made, thus no names are rendered.

The default Carto layer does take population into account, and it can be seen that it is a major factor in deciding which place is shown at lower zoom levels.

In green are cities I’ve recently added population number to (number is brackets is the urban population). These are all rendered as long as the labels fit nicely without overlapping. It is not strange that the desert town of Hami is labeled at this zoom level. It should be, because it is a major stop along this transport corridor.

However, Hangzhou and Xuzhou are not labeled in this case. For Hangzhou it can be argued that the label would overlap with Shanghai, but for Xuzhou this argument cannot hold.

Between 兰州 (Lanzhou) and 天水 (Tianshui) lies 定西 (Dingxi). As you can see here I’ve also added this city’s population number. And although the label would fit, it is not rendered. I have not looked into the code of the Carto renderer, but my guess would be that either it’s population of under 500 000 would not make it important enough, or that the other cities are too close by (Hami is a lot more isolated). For completeness, Lanzhou and Tianshui have a population of a few million.

Also good to note, if you zoom in to level 6, you get a little dizzy from all the city names. Apparently here the renderer shows anything that is a town, and because almost all lack population numbers, all are shown.

It seems that in India for example, most cities do have a population number included, so if you zoom through the levels there, the map looks much more usable.