Photos Using Space on iPhone

I have an iMac, iPhone, and iPad. Each has the latest operating system. I use iCloud Photo Library. I take most of my photos/video with my iPhone. I edit and import to Photos and iMovies, on my iMac.

I have both my iPad and iPhone Photos settings to: iCloud Photo Library-on; optimize iPhoto (iPad) Storage-checked; Download and Keep Originals-not checked; Upload to My Photo Stream-off; iCloud Photo Sharing-checked; and, Summarize Photos-on. My iPhone Storage for Photos & Camera shows I have 35.43G used by my Photo Library, while my iPad shows only 8.1G.

Questions: 1) Why the difference? 2) Since I am using iCloud, and have backups on my iMac and Airport, how can I reduce the space taken up by the the Photos Library on my iPhone?

Thanks in advance for any help.