Think of the VM environment as having a few layers of abstraction.
You have your VM host -> this behaves like the BIOS of a hardware server, where you would allocate the resources available to the destination VM instance.
You then have the VM instance -> this is a pre-defined system that only has the allocated amount of resources.
When installing, you are only working inside the VM instance. At this point, you can choose to either install fresh off the ISO (blanking out only the HD space allocated to that instance), or you can use the OVA which will need to be resized after installing.