In most cases if not all majorities, a large amount of your VPS Diskspace it taken up by the operating system as well as the software installations that you create on your Virtual Machine. There may also be other factors which could include backup and restore files as well as /tmp caching.
To find out where your disk space is being used, login to your VPS over SSH and check the size by the command:
du /* -hs | grep [0-9]M | sort -rn | head -10
This will show the 10 biggest directories in megabytes.
du /* -hs | grep [0-9]G | sort -rn | head -10
This will show the 10 biggest directories in gigabytes.
You can also use other alternatives to current disk volumes like df -u and/or df.
