Windows Blog has published its Windows and Windows Phone Store Trends for December 2014. Among other things, the update shows that 71% of app downloads from Windows Phone Store are from low-memory devices. That would mean devices with 512MB of RAM. These include the following Lumia models: 520, 530, 625, 630, 620, 635, and 720. The Lumia 520 makes up about half of this category of devices.
Here is the chart:
Some really old high memory horses are still in much use, it seems, namely: 920, 820 and 1020. The 930 does not seem to be in much use as it doesn’t show up in a slot of its own but is lumped under “Other”.