Salt size in /etc/shadow

After user password change the size of salt decreased in RHEL/Centos 6, eg:

cat /etc/shadow

...
root:$6$FkMNsNxT$FW77....................nbL0......
bin:*:15422:0:99999:7:::
...

As you can see, FkMNsNxT is 8 characters. Why it happens? In the beginning, after installation, the size is 16 chars.