Texture2DArray Size Problem

Hi! After I Built a Texture2DArray by Texture ImportSetting, like https://docs.unity3d.com/Manual/class-Texture2DArray.html "Importing texture arrays" Section did, the original Texutre is 2048*2048, format is RGBA compressed DXT5|BC3 UNorm. and the size is shown 4.0mb in Inspector PreviewGUI,

After i transfer this texture to Textrue2DArray, the size In Inpsector PrevewGUI became 8.0mb, Is it Right?