Double texture memory usage even if don't check read/write enable

I encounter a memory problem in my game. After using profiler to determine the memory usage, I have found that all of my textures used double memory size. I don’t know what’s wrong here but I makes sure that all my textures do not check read/write enable in the import setting. I tested creating a new scene. I created new plane and applied the material to that plane. I played the scene and the profiler showed that my 1024x1024 texture used 8 MB of memory. It’s a GUI texture so I don’t compress it. It showed 4 MB in the import setting. Even if I compressed it to PVRTC, it still used double memory size from import setting (0.5 MB → 1 MB). What’s the problem? I really need some help here. Thank you for the answer.

Have you turned of MipMapping (TextureType->Advanced->Generate MipMaps)?
Afaik, when using MipMaps Unity3D creates different resolutions (1024x1024, 512x512, 256x256 etc.) for your texture, which results in a higher memory usage.