OS X Photos size different than iCloud Photo Library

I recently decided to move my Photos library on my Mac to iCloud Photo Library. As I wanted to keep all of my original photos and videos also on my mac, I selected Download Originals to this Mac. Prior to starting this process, I wanted to gauge how much space I would need in iCloud, so I followed the following path: Pictures -> Photo Library, and it stated that my library is 182.68 gb (this path was launched from Photos Preferences).

My upload into iCloud Photo Library just completed (after two days), and when I go into Mac Preferences, it shows that iCloud Photos and Videos are only taking up 153.45gb (this is also the value it showed on the progress screen that was being uploaded from my mac).

This is a significant difference (182.68gb versus 153.45), and I am not sure why. I did create a Smart Album in Photos with the condition: Unable to upload to iCloud Photo Library, however the album does not contain any photos.

Is there someway to determine if iCloud has an exact copy of my Photos Library? This is almost a discrepancy of 30gbs..

Thanks in advance for your advice!