as long as you checked expanding disk (the default option), the space used is only the space used. in other words if your windows files are taking up 13GB even though windows shows it's a 65 GB HDD it will only take up 13GB on the mac side. as you add files and your disk space needs increase, the VM file (mac) will grow but only up to the max disk size.
as an example, i have a windows VM that's set to 65GB, but the VM size is 42GB.
you can see how much is being used by doing a Get Info on the virtual machine on the mac side.