Compressed Size

For files, the amount of disk space, measured in bytes, used to store a compressed file's information. 

For folders, the actual amount of disk space, measured in bytes, used to store the files it contains, after taking into account any disk compression.

Note: If a file is not compressed, this will be the same as the file's size.  If none of a folder's files are compressed, this will be the same as the folder's size.