Disk clusters explained

A disk is divided up into "clusters" (or "file allocation units" or whatever they call it now), the smallest unit the filesystem can track. (Which is why we can use larger disks with more clusters with a 32-bit or 64-bit file system.) The typical cluster size today is 64kb. So if you have a 2kb file,  it's actually taking up 64kb of physical space. The "logical space" would be 2kb.

So, different software reports on file size differently, and many of them only report "logical size", not "physical clusters used".

If you have a ton of small files less than the cluster size  (like temp internet files), this will help explain why they use up so much physical space. I'm not sure it would account for a 25GB difference though, but it might.

1. Clean up your internet files and any other temp files.