As long as none of the smaller set of images have been cropped and only are resized, you should be able to do it with Exiftool.
From my brief search, it looks like digikam saves the faces tags to the MWG XMP tags. The basic command to copy would be:
exiftool -TagsFromFile SmallFile -RegionInfo LargeImage
If the filenames are the same, then you could do it in batch with a command like this:
exiftool -TagsFromFile /Path/to/Smalls/%F -RegionInfo /Path/to/Larges
If the names are different, then the commands would have to be altered a bit but it should still be doable.
This command creates backup files. Add
-overwrite_original to suppress the creation of backup files. Add
-r to recurse into subdirectories.
The way MWG regions (face tags) work is X/Y coordinates and the height/width of the region are saved as number from 0 to 1 and used as a percent of the images actual height/width. This means that they will correctly locate the face as long as the image has not been cropped or flipped. Also, it is assumed that the location is relative to the top left of the image as indicated by the
Orientation tag embedded in the image. This allows the regions to be resistant to changes in size, as long as no cropping takes place, and rotation, as long as the
Orientation of the image is properly updated.
For further info, see the MWG Guidelines for Handling Image Metadata pdf on the subject (pages 51-56).