@simonsmark raises a good point. The difference in file size wasn’t +/- 2 MB, as it is in @monster94’s example, when I wrote off the difference to extra metadata. I also had not done anything to the photos except either copy the files (i.e. no opening the file and then saving) or import them once into Photos. And, as I said, the photos weren’t extremely important to me. I also made multiple backups of my unsorted photos before I started culling them so that I could get images back if I had second thoughts about my decision.
Compression is certainly worth considering. It is my understanding that Photos doesn’t compress photos on import. But it’s not clear if @monster94 has had the photos imported into any other digital asset management/photo preview software at some point.
One way to determine if compression, metadata, or both is the culprit may be to look at the average size of your photo files from that camera. Perhaps one file size is obviously an outlier. I also relied on the histogram feature in PhotoSweeper to help me tell if duplicate photos had meaningful differences. As I understand it, compression will show up in the histogram.