Well, the 2048x2048 PNG source is 1.7MB on disk, and the ATF is 1.8MB on disk. So the ATF is not much help here.
In memory, I can store the 2048x2048 PNG texture in 32bpp (8888 color mode) for 16MB VRAM usage, or 16bpp (4444 color mode) for 8MB VRAM usage, which is currently my main format.
The ATF (without mipmaps, just the 2048x2048 texture) appears to use about 25MB VRAM, which does not make much sense.
The Starling manual (https://manual.starling-framework.org/en/#_atf_textures) says that ATF textures:
can be processed directly on the GPU. This means that, depending on the compression settings, you can load up to ten times as many textures
and indeed, if I could use the 1.7MB ATF instead of the 16MB uncompressed texture, it would use 10 times less GPU memory ... so what I am experiencing looks like an incorrect behavior.