When files are compressed on the NTFS volume, Defraggler reports the true size of the files instead of the actual size that it takes up on the disk. Because later on during the defragmenting process it tries to move this big area of files, only to encounter that it has reached the end-of-file point laid out on the disk sooner than expected, and moves on to move the next file in line. This is exceptionally noticeable of large files, especially large text files whose sizes were about halved using drive-level compression.