Photo Stream imports smaller file

Photo Stream automatically imports photos to Photos on iMac, but when I connect iPhone to iMac, all photos not previously manually imported show as new, and then when I import them to Photos, they are a larger file size than the automatically imported photos from Photo Stream (which, I presume, is why Photos sees them as different photos than the Photo Stream photos already automatically imported). The files look exactly the same and have the same resolution, but I presume the smaller file size versions must be optimized (with, again I presume, some loss of quality). This started happening only recently.

How can I make Photo Stream photos and manually imported photos recognized by Photos as the same photo, so I am not prompted to re-download photos I already have in my Photos library?