We took a 410 MB game build and looked at where the bytes were: textures 268 MB, audio 74 MB, meshes 31 MB, code 22 MB, everything else 15 MB. Changing compression on the APK moved the download by 12 MB. Changing the texture format moved it by 180 MB.
ASTC at 6x6 with a proper mip chain replaced a mix of PNG atlases and ETC2. Visual differences were invisible on a phone screen at normal viewing distance for the art in this game, though we would not apply the same setting to UI text.
The lesson is boring and worth repeating: measure the contents before tuning the container. A zip deflate level change is a rounding error next to a texture format decision, and the format decision is reversible if you keep the source assets.
Read next: Packaging: APK, AAB and the size budget.