I could not find the option, but then discover it by switching 'UPLOAD SIZE FOR PHOTOS & VIDEOS' to Original, then switch back to High quality (Compressed). Then it will prompt with message how much storage used by uncompressed photos can be compressed to High-quality, all you have to do is confirm there.

I could not find the option, but then I discovered it by switching 'UPLOAD SIZE FOR PHOTOS & VIDEOS' to Original, then switch back to High quality (Compressed). Then it will prompt with a message how much storage used by uncompressed photos can be compressed to High-quality; all you have to do is confirm there.