On my installation I see the following in the Gallery Statistics block: Disk Usage: 1.6GB
However the size of the directory data/xengallery is 2.6GB. And I think XFMG also stores copies of its media on other folders inside data and also inside internal_data.
It doesn't aggregate the file size from the file system, it gets them from the xf_attachment_data table. It sums up the file_size field of the media items as cached in that table.
So, it doesn't count anything in the data directory.