If a virtual machine is not actively using all of its currently allocated memory, ESXi charges more for idle memory than for memory that is in use. This is done to help prevent virtual machines from hoarding idle memory.
The idle memory tax is applied in a progressive fashion. The effective tax rate increases as the ratio of idle memory to active memory for the virtual machine rises. (In earlier versions of ESXi that did not support hierarchical resource pools, all idle memory for a virtual machine was taxed equally.)
You can modify the idle memory tax rate with the Mem.IdleTax option. Use this option, together with the Mem.SamplePeriod advanced attribute, to control how the system determines target memory allocations for virtual machines. See Set Advanced Host Attributes in the vSphere Web Client.
In most cases, changes to Mem.IdleTax are not necessary nor appropriate.