Photos Blurry When View Size Increased

It does not occur on all photos but one thing consistent ... it occurs on the older photos that were migrated from iPhone to Photos when Apple did the change.

What is the image format of these old iPhoto photos?

After the Catalina Upgrade some older codecs have been deprecated and Photos can no longer process all image formats and video that we could use previously.

You may want to check the format of the blurry photos in Photos. Select one of them and try to export the photo with "File > Export > export unmodified Original". Then open the original in Preview. Does it look alright in Preview or is t blurry as well? Check the width and d height of the photo in pixel. Does it still have a decent size or is it just a tiny thumbnail?

On problem with the photos migrated from iPhoto is, that Photos cannot process the photos directly, if the photo has been edited in iPhoto. In that case you could try to open the blurry photo in Edit Mode and use the command "Revert to Original". Does that remove the blur?

Have you by any chance been running a cleaning application to remove duplicates from the library (from the iPhoto Library or the Photos Library)? Some of these programs will damage the library and you may be left with blurry previews.

Ok you could try to reset the photos app on your mac.

Before doing so make sure you have a copy of your photos somewhere else.

Then Close Photos.

Find the Photos library that you have just created. It's probably in your User>Pictures folder. Drag it to the Trash, and empty the Trash.

Open Photos while holding down the option key. It should then give a list of the different iPhoto libraries you can import. House the one you want.

Are the photos blurry too when you view them on other devices than your mac? What about in other applications than the photos app for mac?

The current photo library (the one were the photos are blurry) but please do make sure that you have a backup of those photos before deleting it.

Mhhhh yes, if you have a copy of those original photos somewhere I would try to replace the "corrupted" photos in your iCloud photo library with the orignal photos.