With the Windows VM being the only VM running on the host, the host will allocate physical memory pages for the VM as it requests/touches memory. Windows likes to initialize memory on boot and other occasions, which it later does not necessarily in use (from the guest point of view). However, once allocated, the host does not know which memory pages are in use by the guest and which aren't. If you would power on another VM on this host now, VMware's memory techniques (TPS, ballooning, ...) would start to do their job.
André