[Libguestfs] Real size vs virtual size image question?

Hello All, I have been doing some reading and it seems that for some images created that the "real" size is not the same as the reported "virtual" size to the filesystem. There actually is a name for this, but I cannot remember what it was now and think that I ran across it in my reading on the Libguestfs site somewhere. Anyway, I have some images that have actual sizes around 4 MB but are reporting sizes of 1 TB. I would like to compress and zip up these images but based upon the 4 MB size and not the 1 TB size. Is there any simple tool that will do this, or perhaps some source code that I can compile into a tool or library that will do this as I ultimately need some simple C/C++ or Golang solution to compress and decompress these files properly? Any ideas or suggestions would be greatly appreciated. Best Regards, Lonnie Email: Lonnie.Cumberland(a)Outstep.com