http://is-it-true.org/nt/nt2000/atips/atips70.shtml

Directory Information Tree (DIT). In NT 4.0, the Security Accounts Manager (SAM) database holds all the user, computer, and group information for a domain. Because the SAM is a Registry hive, the Registry's scalability limits the SAM. On Win2K domain controllers, the DIT replaces the SAM. The DIT is based on Microsoft's Jet database engine and is similar to the Jet engine that Microsoft Exchange Server uses. The file ntds.dit, in the \%systemroot%\ntds directory, is the Win2K equivalent of the SAM file. This file stores the bulk of the directory database. In general, the DIT is larger than the SAM because AD holds more information and types of objects than NT 4.0's directory service holds. Within a domain, the contents of the ntds.dit file replicate to all the domain controllers. You might think that you'll see more replication traffic between domain controllers when you migrate from NT 4.0 to Win2K. However, Win2K uses a completely different model for replicating directory changes than NT 4.0 uses.

http://www.netpro.com/forum/messageview.cfm?catid=11&threadid=129

http://www.microsoft.com/technet/treeview/default.asp?url=/technet/prodtechnol/AD/windows2000/maintain/adsize.asp