Are Google Photos' high-quality pictures compressed even if they're under 16 megapixels?
I just took a picture with my phone (8MP) after configuring auto backup with both Google Photos (with "high quality" option enabled) and Dropbox.

I compared both images and I can't see a difference between them, although the file sizes differ (the image in Google Photos has 1.3 MB and the one in Dropbox has 2.2 MB). I made the same comparison with an older image (when I had the "original" option enabled in Google Photos), and the files are exactly the same.

Seems like Google is really compressing the file, although keeping the image quality.