Hi,
According to this : VMware vSphere 4 - ESX and vCenter Server
"ESX/ESXi provides a memory compression cache to improve virtual machine performance when you use memory overcommitment. Memory compression is enabled by default. When a host's memory becomes overcommitted, ESX/ESXi compresses virtual pages and stores them in memory.
Because accessing compressed memory is faster than accessing memory that is swapped to disk, memory compression in ESX/ESXi allows you to overcommit memory without significantly hindering performance. When a virtual page needs to be swapped, ESX/ESXi first attempts to compress the page. Pages that can be compressed to 2 KB or smaller are stored in the virtual machine's compression cache, increasing the capacity of the host.
You can set the maximum size for the compression cache and disable memory compression using the Advanced Settings dialog box in the vSphere Client "
Memory compression does not reserve memory, it is only a process provided from your host to compress memory from 4KB to 2KB.
Regards,
Julien