iCloud Photo Library not uploading entire Photos library

Hello,

I decided to start using iCloud Photo Library, so I ticked the option under "Preferences" -> "iCloud" in Photos (version 2.0). It started uploading items fine, but I noticed that it is not uploading the entire Photos library. My photos library contains 7065 photos and 151 videos, but Photos only started uploading about 2500 items to iCloud Photo Library after enabling it which is 16,5 GB (see attached screenshots). When looking at the size of "Photos Library.photoslibrary" it is 46.5. Does anyone know a reason for this difference? Thanks