The size of the Active Directory increases rapidly on a Windows Server 2008 R2-based domain controller that hosts the DNS Server role.
The issue occurs because of incorrect Active Directory tombstoning of DNS objects. DNS clients should reuse the existing DNS objects that are marked to be deleted when they register DNS records. The existing DNS objects are usually referred to as “reanimating objects.” However, currently the DNS server service creates a new object for these DNS clients and moves the existing DNS object to the deleted objects container. Over time, this behavior causes the Active Directory DIT file size to increase significantly.
Hotfix Info: http://support.microsoft.com/kb/2548145