/proc is a seperate filesystem. Just like /usr ot /var is a seperate filesystem. /var is a mount point. Do a "du -sk /var/*". I'll bet that you find some files there. Do you understand that the files in /var have no bearing at all in the space consumed in root? /proc is the same way. It is a mount point. What's more the filesystem that is mounted there is not real. If you can't grasp that, just ignore it. You don't look in /var for files to delete when root is full do you? Well don't look in /proc either. Or /usr. Or /tmp. And so on. You need to find files in root to delete. Removing a file from /var does not return space to the root fileststem. Neither would /proc.

If you want more space in your root filesystem, you must delete some files that reside there. Deleing files in other filesystems won't help. The space in root is consumed by files that reside in root. When a file resides somewhere else it doen't eat up space in root.

As for how it works, type:
df -n / /proc
and you can see the filesystem type. When you open a file on / the open system call see that / is a ufs filesystem. So it calls the ufs code to open the file. The ufs code really does read a disk somewhere to get its info. On the other hand, when you open a file in /proc, the open system call sees that the file is in a proc filesystem so it calls the proc code. The proc code scans the currently running processes instead of reading a physical disk. And the read, stat, and write system calls do the same thing. This is called the filesystem switch. It was first used to put nfs into the kernel.