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.
Thank you for the suggestion and running the test. I changed the Preview settings to 1440, and then 2560 and finally, Don't Limit. Quit and relaunch Aperture between each setting. No luck.
But you gave me a hint: maybe the problem has to do with the images being first Managed and then moved to Referenced files. So I tried one that has never been a Managed file and everything works correctly! Don't know if this is a bug or a feature.
Mystery solved, but I still have a lot of images to reprocess.
Thank you for the help.