Look at the final field on your df listing. Those are your separate filesystems. Stuff in /proc, /usr, /var, /zone_1, and so on is not in /. And stuff in /proc is nowhere except in memory. As you say, it is your pids. You can delete stuff in /proc by killing processes, but this will not help you. /proc is not a real filesystem, it is a way to access your processes as if they were files.

/ should not grow very much. Has it grown in, say, the past 10 days? Then get a list of files in / modified in the past 10 days...
find / -mount -type f -mtime -10
This will probably be a short list and it should tell you where the problem is.