When you install Proxmox it creates two partitions, one is for /boot the 2nd is for pve LVM.
In LVM it sets up three volumes:
swap
/
/var/lib/vz
I am assuming that you are seeing the storage in the Proxmox GUI as being 68GB, that is /var/lib/vz
The rest is in swap, / and /boot with a portion also left over in the LVM group being used so Proxmox can make snapshots.
You can control how the partitions are setup when starting the installer.
When the installer first starts instead of pressing Enter, type "linux" followed by the following options(s):
maxroot=XX
swapsize=XX
minfree=XX
maxvz=XX
XX is an integer value representing Gigabytes
If you wanted the most space while still having a little bit of swap I would suggest:
linux maxroot=10 minfree=16 swapsize=4
You should end up with about 97GB on /var/lib/vz with a 128GB disk
If you are not planning to use snapshot backups you could reduce minfree too, but I *think* 16GB is the smallest the installer uses even if you set it smaller.
