It's inappropriate to target space saving when it comes virtualization, as it's not what hypervisors are designed for - if so, a hypervisor would have to interp

It's inappropriate to target space saving when it comes virtualization, as it's not what hypervisors are designed for - if so, a hypervisor would have to interpret the underlying FS calls, and optimize them space-wise. Besides, I think pretty much all the formats behave the same; qcow2 for example, does not optimize on the fly the writing of zeros. All in all though, if you ultimately moved to a native Windows 10, therefore allocated X space of native disk, the space requirement is not different from having only a virtualized one with X space of virtualized disk (heck, you could even pass a partition). Having said that, if 10-20 GB of space are taken so quickly on boot, it may be worth investigating (you could even vim the diff disk), but I find your claim somewhat misleading, as you're likely experiencing this on the first absolute boot after taking then snapshot, but not on the subsequent ones; therefore, if your system steadily takes 20+20 GB of space, that's a perfectly reasonable requirement.