My Photos & Camera storage decreased drastically, but how?
The "Optimize [device] storage" setting, associated with iCloud Photo Library does not work instantly, so it may explain the behavior you describe. It needs to upload your originals to iCloud and then it may kick in only if you seem to need space on your device.

From an article on Macworld:


  If you take a photo with your iPhone (or other iOS device) camera, iOS doesn’t downsample or optimize images or video until it’s uploaded it to iCloud Photo Library and the storage is needed on the iOS device.