Move VM (VM) to second harddsik [noob alert]

Hi there,

we have a proxmox 3.1 server with 7 KVM-VM's. We added a second harddisk and via the gui moved 2 of the VM's to the second harddisk. But get moved over fine, but their size is the actual size of the HD space allocated to the VM's. I was under the impression the VM HD's when a new KVM is created are incremental?

What do I miss please, any guidance to a noob is appreciated.

Regards,
-HF