UNIX for Advanced & Expert Users

I have unix solaris , size of root directory decrease continuously like :
'/ : Disk space: 13.92 MB of 100.45 MB available (13.86%).'
and decrease more and more ,
How can i detect the reason ?

I know that English may not be your first language, but the terminology is important. You are asking about the root filesystem, not the root directory. /etc/passwd is in the root filesystem but it is not in the root directory.

Here is how to get a list of all of the files in the root filesystem that have been modified in the past week:

find / -xdev -type f -mtime -7 -print | xargs ls -l

Since you seem to have one or more files growing this command should find them.