Google Photo Compression
The follow is a 'Get Info' of the same file one was upload to Google Photos and the other to Yahoo Flickr back in July 2015. I now wanted the picture to create a calendar on Shutterfly:
As you can see the file that I downloaded from Flickr is much bigger than the one from Google Photos. It also means that the quality is going to be better.
Normally this isn't a concern, as most of my pictures are just sharing online. However, in this case, it is an issue because I wanted to add the Photo to a calendar on Shutterfly. Shutterfly was throwing warnings that the picture quality is low and the picture image might be pixelated.
This is why you shouldn't be using Google Photo as your main source for photo cloud storage, at least if your only using the 'High Quality (free unlimited storage)' option. Google says is "Great visual quality at reduced file size." This isn't a great option if you plan on doing any printing of your photos.
Google Photo iOS App
If you are using Google Photos app to back up your iPhone 6s Plus photos, the will also get reduced to a smaller size:
In this case, my file was reduced by only 100 MB, which isn't all that bad for about 99% of the photos that I take. I just need to remember for that 1% of the photos that I really like, to keep a better quality some place else.
You could upgrade Google Drive storage to 100 GB at $1.99/month (about a cup of coffee) and upload your photos at the Original quality. This means that I would be able to store approximately 33,333 photos from my iPhone. Which isn't all that bad.