I should probably just leave this to someone better informed to answer, but /dev/null is used in UNIX to discard output from a command when you don't care what it produces. In your case, it's a file called devnull (or, more accurately, ~/devnull since it's in your home folder), so I suppose it's possible that something you installed had a typo that accidentally sent stuff to devnull rather than /dev/null, which would rapidly fill up a file with junk over time.
If that's the case, the file has nothing but junk in it, and the only way to keep it from being recreated would be to figure out what's making it and turn it off.