Are Google Photos' high-quality pictures compressed even if they're under 16 megapixels? Recently, Google has changed the Google Photos app so that everyone has free and unlimited access to store 16 megapixel files.

The camera on my phone is under 16 megapixels. When I downloaded the app, it asked me whether I wanted to keep the photos in original or high quality.

I would like to take advantage of the free storage so my question is if the photos are originally less than 16 megapixels, is high quality going to be the same as original or would Google compress it more?

Also, what happens to the existing photos on my Google Drive that are less than 16 megapixels? Are they automatically not counted toward my quota or do I need to somehow migrate them to Google Photos? Just that, the majority of the files on my Drive are photos (which are less than 16 megapixels) and since I've downloaded Photos, the space available has not changed.
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.