VirtualTexture Disk Size? (SVT)

I have a Landscape of 4 kilometers x 4 kilometers, and when I use RVT and CVar 'stat virtualitexturememory’to check memory usage, the Total Disk Size is 89MB. Then, when I use Build SVT and set Level 7, the Total Disk Size becomes 1178MB! WTF? The SVT file(Virtual Texture Builder) only has 13MB. I want to know if this is just a bug or really happen

Not a bug, RVT computes what is necessary to render the current view so it is much smaller. Building the SVT will render the entire world to a texture saved on disk up to the set mip-level.