Situation
Virtual CentOS 6-guest doesn't return memory to host.

Resolution

The guest will keep the existing claimed memory until either the guest is rebooted, or memory ballooning takes place.

 

Source

http://serverfault.com/questions/654738/hyper-v-centos-6-6-guest-with-dynamic-memory-should-i-see-memory-decrease-fro

https://www.centos.org/forums/viewtopic.php?f=13&t=45334

https://social.technet.microsoft.com/Forums/windowsserver/en-US/21519c00-47d4-44a4-b2d3-64cd3098849e/rhel-70-centos-70-rhel-65-centos-65-rhel-64-centos-64-include-support-for-hyperv?forum=linuxintegrationservices