VMmanager supports the following formats for virtual disks of virtual machines:
RAW is a data format with unprocessed data or data with minimum processing. With this format, a virtual machine takes as much space as it was given during its creation. It is possible to convert storage from RAW to Qcow2. You can do it by following the article Converting disk format from RAW into Qcow2.
Qcow2 is a disk image format of QEMU. A virtual machine takes as much space as it has data on it.