Irreversible data loss is a sad story. John Marvin writes in with a question about restoring images that he exported:
I exported a number of important photos to a back up hard drive and then removed them from Photos as they were not regularly needed. When I then opened up the photo from the HD it was substantially reduced in size. How do I restore the original size of these photos?
If it’s been less than 30 days since you deleted the images from Photos, they may be in the Recently Deleted album in the sidebar. (Choose View > Show Sidebar if you aren’t seeing a list of albums at left.)
However, unless you have other backups (like Time Machine or external drives or cloud-based copies), the full-resolution images are gone for good.
What likely happened is that in using the export option in Photos, the choices were set to a lower resolution than truly desired.

The default export option in Photos hides that you might be reducing resolution.
In Photos, choosing File > Export > Export [x] Photo(s), the export option is collapsed by default, showing just Photo Kind. I don’t have a default installation, but when you click the downward-pointing area to expand the Photos section of that dialog box, the Size menu isn’t set to Full Size.
If you use the default options in collapsed or expanded view, you’re thus exporting a downsampled/lower resolution version of your original. Full Size incorporates any changes you made into a photo (or video), although it’s still doing some processing.
For media you’ve just imported into Photos and haven’t modified at all, choosing File > Export > Export Unmodified Original for [x] Photo(s) copies exactly what was imported into Photos.
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