Some of my log files have grown to excessive size:

-rw-------  1 root  wheel  36183077 Mar  2 22:44 /var/log/maillog
-rw-r--r--  1 root  wheel  23935707 Mar  2 08:39 /var/log/messages

Even when I am logged in as root, I cannot move or delete these files. 
I have tried newsyslog and it returns no errors, but the files are 
unchanged.

What is the trick to moving or deleting files in /var/log?

Can anyone help please?