What to do with old iPhoto library after migration to Photos

Thanks for your reply but I'm still confused as to how I lost over 10Gigs of HD space after the migration. I understand the conversion is supposed to create more space, not less. When I look in the pictures folder in Finder, I see 2 LIbrarys, iPhoto and Photos, each of them approx 15 Gbs. according to the Get Info menu. I guess what I don't understand is if the iPhotos Library is not needed and is moved to an external drive why that wouldn't free that amount of space.

Also, if "optimization" is supposed to move "some" high res images to the cloud to make room on the HD, how is it that I find myself with only 1gb left on my HD? What is the criteria that triggers "optimization" and how much room can we expect it to shoot for? There seems to be a variety of answers on this subject, many of them sound like guesses. Apple's explanation is vague about how it works, so hopefully someone has the accurate, in depth scoop on this.