Tom, can it be, that your referenced files are offline? Dragging and dropping will only export the previews and the full versions.
I checked with referenced on the same drive and managed. If the "Preview" settings in Aperture's Preview preferences are set to "half size" previews, then I am seeing the same.
If the originals are referenced, aperture will drag and drop the smaller previews, but if the originals are managed, Aperture will drag and drop the original file.
You may want to change your "Preview" preferences to "fit within 1440 pixels".
Then your previews will have the size you want.