...
Understanding Dynamic Memory...
Windows zero's memory on boot so uses it all.
Some Linux (check Ubuntu and how LXD goes into this) is smart with balloon drivers which release memory back to host if not used.
References
Hypervisor view of managing resources - http://www.altaro.com/hyper-v/common-hyper-v-deployment-mistakes/
Q&A... Memory aspect was new to new to me - http://www.linux-kvm.org/page/FAQ