System administrators use three basic parameters to control the allocation of memory to each VM: a min size, a max size, and memory shares. The min size is a guaranteed lower bound on the amount of memory that will be allocated to the VM, even when memory is overcommitted. The max size is the amount of ``physical'' memory configured for use by the guest OS running in the VM. Unless memory is overcommitted, VMs will be allocated their max size.
Memory shares entitle a VM to a fraction of physical memory, based on a proportional-share allocation policy. For example, a VM that has twice as many shares as another is generally entitled to consume twice as much memory, subject to their respective min and max constraints, provided they are both actively using their allocated memory.