The reported size isn't very precise but close enough to get an idea of
what you will get. The problem is that raw files always have some unused
regions that get cropped away later.
Another source of discrepancy can be manual cropping by the user in
darktable, but there isn't much we can do about that.
Before there wouldn't be any intermediate file being written when
darktable sees more than one XMP file. Now the first image in its
temporary database gets exported.
Tobias Ellinghaus