Try this experiment.
1. Import a camera produced JPEG into Photos.app. Check the size. Let’s say it’s 6.8MB.
2. Don’t edit the image. Export it as maximum quality JPEG. Check the size. It will be something like 17.2MB.
Even though the image was native JPEG, and it wasn’t edited, Photos.app decompressed and then recompressed it. Adding 11MB of non-value.
Unchanged from iPhoto and, for that matter, Aperture.
There should be a better way. (No, unmodified original is not a better way — because if editing were done, or the original were not JPEG, then you would want the transformation.)