Windows still reports a high memory usage in the Task Manager even while Windows is idle after the memory intensive application in Windows was closed.
Some Windows memory intensive applications may accidentally crash.
The Memory of the virtual machine is set to a yellow-red zone in the virtual machine configuration > Hardware > CPU & Memory
I run a memory intensive application in Windows, then I stop it but the virtual machine physical memory size used by Windows is still high:
All the Memory assigned to your virtual machine is virtual, and there is no guarantee that the virtual machine gets exactly the same amount of Mac physical RAM as its virtual RAM size.
When the virtual machine is provided with Memory, its maximum size cannot exceed the physical RAM installed on the Mac. Moreover, no matter how much Memory is assigned for the virtual machine in the virtual machine Configuration, some significant amount of Mac RAM (usually not less than 2 Gb) should still be left for the OS X to guarantee its performance and stability.
That is why Parallels Desktop manages the virtual machine memory assignment in a way that it sometimes "steals" memory from a virtual machine in order to meet the OS X (host system) memory needs.
The high Windows memory usage inside the virtual machine indicates that this kind of "theft" was done.
The workaround below should only be applied if you really need to have exactly the same amount of virtual RAM in your virtual machine as you specified in your virtual machine's Configuration. For example if some specific Windows application requires the certain amount of RAM in Windows.
The workaround could probably affect the virtual machine's performance.
Open your virtual machine configuration > Hardware > Boot order.
Insert the following boot flag into the Boot flags: text field:
Now the memory size used by Windows inside the virtual machine will not grow in the idle state: