Question: Why does treesize showed that my files
occupied only 637 MB of space, but chkdsk showed they occupied
1279 MB?
Answer: Chkdsk shows the space used as well as space
wasted. You must be using the FAT disk format. In this
format, each unit of space is called a "cluster", which can be
as big as 32K. Even the smallest (1 byte) file will occupied 32K. In
the above example, almost half the disk space is wasted this way.
You can reclaim about half the wasted space if you reformat
your disk to use a smaller (16K) cluster size. If you are running
NT, use the NTFS format, you won't waste any space there, and it
will be faster too.