ESX Server supports overcommitment of memory to facilitate a higher degree of server consolidation than would be possible with simple static partitioning. Overcommitment means that the total size configured for all running virtual machines exceeds the total amount of actual machine memory. The system manages the allocation of memory to VMs automatically based on configuration parameters and system load.
Each virtual machine is given the illusion of having a fixed amount of physical memory. This max size is a configuration parameter that represents the maximum amount of machine memory it can be allocated. Since commodity operating systems do not yet support dynamic changes to physical memory sizes, this size remains constant after booting a guest OS. A VM will be allocated its maximum size when memory is not overcommitted.