Graphic resources size in builds

Hello! I'm working with Unity for two months and I really like this time-saving framework, but still there is one thing that puzzles me. Why does Unity save graphic resources in Android apk's and iOS ipa's in a bitmap format? I want to save my resources in a small apk, but instead of 1mb for an image Unity stores 11mb for a single image. Maybe I don't fully understand some aspects of a cool game development, but it seems to me that other frameworks are clearer in this aspect. I would be grateful for any information.

Best Regards, rd.