Using xml to copy large and complex data sets is not recommended by Esri.
The Help entry "Copying a geodatabase using geodatabase XML workspaces" notes:
<quote> The resulting geodatabase XML document can be huge when you copy both the data as well as the schema. Generally, this alternative is not recommended for copying geodatabase data. Alternatives for making a geodatabase copy include the use of the Copy tool and the Extract Data Wizard in ArcMap to copy subsets of the information. <unquote>
Exporting to xml depends on how well the workspace contents render into xml by Arc, which is expecting arc-type objects.
Things it cannot handle (BLOBS like attachments) get stored as XMLPersistedObjects, which are, I imagine, a bit unwieldy.
The Copy tool is the primary method.
Try reading: Help > Geodata > Geodatabases > Copying geodatabases