Exporting an image at less than full size is very blurry

I've done enough experimenting to sort out the issue. My out-of-camera jpegs are VERY large - 12 M or more. My camera has a large sensor and I have it set to the highest quality setting. Photos loads and displays these fine, but it is having problems exporting a downsized version of these large files. When I load smaller jpegs to Photos, it can downsize and export them fine.

I have the issue no matter what quality I use when exporting any downsized image. All quality settings look okay when I export actual size.

I am using Apple's Preview. I was tipped to the issue by several recipients of an email from me with attached photos. I've been experimenting all morning now with the Export function of Photos. Any export size other than "Full Size" is laden with digital artifacts. They look absolutely fine in Photos, but the exported file is trash. Yes I can export full size with JPEG quality set to low and get an acceptable image of manageable size. But this shouldn't be necessary. I suspect that the problem has to do with the files loaded from my camera. They are jpegs not raw files, but they are very large because my camera has a relatively large sensor and I have set the camera to record them as 'fine' quality. The initial jpegs are typically over 12 Mb - large from out of camera jpegs. For whatever reason, Photos is not doing well when trying to downsize them. I had been using an earlier version of my camera (a Fujifilm X100) that had a much smaller sensor and Photos seemed to export them just fine. I am going to experiment some more and confirm that Photos was downsizing the old camera's images well.

Yes that is the problem. The original out-of-camera jpegs are very large. They load fine to Photos and they look fine in Photos. But when exporting the images, any size other than actual is laden with artifacts. Evidently the rendering software that Photos is using is throwing up when asked to resize a natively very large jpeg. This is a bug.

The old Aperature software did a far superior job of maintaining image quality even when downsizing. What Photos produces when I export anything other than full size is an embarrassment.