Why is size of the ntds.dit file different on different domain controllers?

The ntds.dit file contains Active Directory (AD) information, and because all domain controllers (DCs) replicate AD, you might expect the file to be the same size on all DCs. However, you might find differently sized files because the database file is created individually on each DC, and the data--not the database file--replicates. Thus, several factors can lead to files with different sizes:

Over time, the database can become fragmented. Although objects are deleted while the database is online, you can't compact the database online (compacting requires a manual offline defragmentation).
If a DC is a Global Catalog (GC), it contains information about objects of other domains in the forest, thus making it larger than non-GC servers.
The displayed size of the ntds.dit file is the size of the file when you started the DC; if many objects are added and you restart one DC, that DC's ntds.dit file size will appear larger.

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