File is too big for /dev/null
It looks like your /dev/null may have been deleted at some point so when you started writing to it you are writing to a plain file rather then the character special null device.
You an confirm this by looking at the output of ls
$ ls -l /dev/null
crw-rw-rw-. 1 root root 1, 3 Sep 28 08:11 /dev/null
If I'm right then you won't see a character special device. You can repair this by first deleting the file you created and then running
MAKEDEV std
which should work on EL6 or
mknod -m 666 /dev/null c 1 3