So Google Photos has free unlimited storage. Not. That's a whole another discussion. This post is to cover what to do when you discover that you are using too much of your free 15GB of Google storage for photos.
Here's where to look for the problem.
Now I'm not really having a problem but my daughter did.
I believe there's an underlying problem with Google wrongly counting older photos against your storage even if they are under the 16 megapixel size.
This post from the Nexus 5 subreddit gave me the solution.
WARNING: The following procedure will resample your full resolution photos down to 16 megapixels.
Go to Google Photos Setting screen.
Click on "RECOVER STORAGE".
And click on "COMPRESS".