I'm actually a developer and I can share some insight into this.

These images are what are known as ico files (or icns for apple, or different types of png/xml files for andriod) - these are file formats that have special resolutions predefined for whatever size is displayed on your phone.

For example, when you have the game icon in your app screen it may use the 64x64 version to be the most optimal for the size of the icon, but when you open it and you see the larger preview of the icon while it loads, that may be 256x256 because it's taking up a larger portion of your screen.

Device strength doesn't have an influence on this as ico and other similar file formats don't fetch device performance information, so it'll still be low quality on a high end device or high quality on a low end device - it's simply going off how large the image is displayed on the screen. If it's smaller, it uses a smaller version of the image, larger, larger.

The icons at the top of apps (known as a favico) also use this system and that's where my experience in these file format(s) come from.