i have a test, the same 2048x2048 texture
bubbles_05.uasset 461kb
export to a .tga file 16,385kb
open with photoshop and save as png file with default settings. 213kb
So, it seems Android(ETC1) use tga format. if this is limited by hardware, but i think if editor use png format directly will be a good choice, i mean .uasset file contains source png data directly if asset is texture.
as you see: a 213kb png file store in 461kb in editor, and package to 16MB in android(etc1).