If you click on the cluster, then the virtual machines tab or on any virtual machine you will see a row of tabs with details on about performance. The below 3 give very accurate memory statistics which can help with future planning or even seeing where a performance problem lies

Memory Size -MB

The amount of memory given by an admin to the machine initially on build

Host Mem – MB

The metrics here is showing you how much memory a particular VM is consuming from the ESX(i) host that it’s being hosted on

Guest Mem – %

This is just a metric to show you how much of that memory is actually being actively used from the overall allocated memory.

VMware Memory Resource Management Doc

Understanding Memory Resource Management in VMware® ESX™ Server

Further explanation

What tends to confuse people is a rather high consumed host memory versus a low active guest memory … usually followed by the question on how exactly active guest memory is calculated.

1) Why is consumed host memory usage higher than active guest memory? (p.5)

“The hypervisor knows when to allocate host physical memory for a virtual machine because the first memory access from the virtual machine to a host physical memory will cause a page fault that can be easily captured by the hypervisor. However, it is difficult for the hypervisor to know when to free host physical memory upon virtual machine memory deallocation because the guest operating system free list is generally not publicly accessible. Hence, the hypervisor cannot easily find out the location of the free list and monitor its changes.”

So the host allocates memory pages upon their first request from the guest (that’s why consumed is less than the configured maximum), but doesn’t deallocate them once they are freed in the guest OS (because the host simply doesn’t see those guest deallocations). If the guest OS re-uses such previously allocated pages, the host won’t allocate more host memory. If the guest OS however allocates different pages, the host will also allocate more memory (up to the point where all configured memory pages for the specific guest have been allocated).

2) How is active guest memory calculated? (p.12)

“At the beginning of each sampling period, the hypervisor intentionally invalidates several randomly selected guest physical pages and starts to monitor the guest accesses to them. At the end of the sampling period, the fraction of actively used memory can be estimated as the fraction of the invalidated pages that are re-accessed by the guest during the epoch”.