When you run Disk Defragmenter on a compressed volume, the Show Details screen shows the system cluster size changing from a uniform size to varying sizes after Disk Defragmenter is 50 percent finished.

When you run Disk Defragmenter on a compressed volume, you see uniform boxes on the Show Details screen until Disk Defragmenter is 50 percent finished. After 50 percent, you see boxes of various sizes. This occurs because Disk Defragmenter is defragmenting the actual physical hard disk during the first 50 percent, and the compressed volume during the second 50 percent.

You see boxes of different sizes because of the way Disk Defragmenter reflects how files are stored on an uncompressed drive and on a DriveSpace compressed volume.

For additional information about clusters and DriveSpace, Please see the following articles in the Microsoft Knowledge Base: