I have just compared originals I had which were uploaded to Google Photos vs the ones downloaded.

Visually there is a barely perceptible difference in a side-by-side comparison on a high quality and calibrated monitor. I wouldn't have noticed if it weren't side-by-side and zoomed in.

On the file size, it's gone from a 6MB jpeg down to about a 1.5MB jpeg.

On the smaller files that difference is far less, but jpegs above 3MB seem to see the biggest reduction in file size.

Thankfully I have all of my originals that weren't taken on the phone camera so I don't mind this so much as running my exiftool script over the original archive produces the same naming convention and I can merge those folders and still have the originals I care about.

Sheesh though... such a thing for Google to screw up on.