ESXi provides a memory compression cache to improve VM performance whenever you use memory overcommitment. Memory compression is enabled by default. ESXi compresses virtual pages and stores them in memory when a host's memory becomes overcommitted.
For more information, take a look at the following PDF: https://pubs.vmware.com/vsphere-51/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-51-resource-management-guide.pdf.
By default, up to 10% of VM memory can be compressed—the advanced system parameter Mem.MemZipMaxPct determines this value. The value is a percentage of the size of the VM and must be between 5 and 100 percent.
Compression is enabled by default, but if you want, you can disable the memory compression mechanism by using the Mem.MemZipEnable advanced system parameter.