Resizing disk failure on Cloudmin KVM version

I have Cloudmin KVM version, the OS is sitting on a 16GB hard drive, and an NFS storage mapped to the /kvm folder (where I keep my VPS’s) as well as /var/webmin/server-manager (where I keep my template images).
The free space on the OS is around 12GB and I still have plenty of TB on the NFS storage free to use.
I want to expand/resize the disk on few VPS’s to by at least 20GB, however cloudmin doesn’t allow me to create a greater HDD size for the VPS than the free space left on the OS (which is 12GB) the error given is “Failed to save disk : Creating or resizing this disk would exceed the host’s free disk space of 12.57GB”.

I’ve tried to do his on cloudmin Xen based server (exact same configuration with the same NFS shares mapped but a XEN based system) and the creation or resize of HDD bigger than the free space left on the OS is successful (as it’s being added on the NFS share not on the system OS disk.

How to get this working? is there a configuration I’ve missed?

Thanks for any help.