My 2 years free Terabyte of Drive Storage from Chromebook ended and I was surprised to be so full. Doing a little digging I figure I can move some stuff to my other storage but the key was to get those full resolution backed up photos resized to the unlimited storage size.
The only reason they weren't already reduced was because G+ had a habit of changing the backup setting with each update from unlimited to full without warning. I have since found where to compress the backed up photos and should see an improvement.
Of course, it did warn me that this process will take over an hour, so we'll see.