Build/apk size doesn't change

Hi guys, I started my first Unity mobile project - very simple game, but the apk is 22 MB big. So I checks what was going on and found that I hadn't compressed any of my textures, yet (2 sprite sheets, each 10.7 MB). So I managed to compress them down to 2 MB, BUT the apk didnt change by 1KB - still 22 MB.

I'm neither artist, nor programmer. Can someone help me?

The sprite sheets only contain true white and transparent - I used the "RGBA Compressed PVRTC 2 bits" format.

Thanks alot.