53 GB in iLifeAssetManagement folder but only 5 GB in Photo Stream
The sub folder contains all the legacy photos from iCloud and iPhoto and normally should have photos and videos. 

With 10.10 and the new Photos app, everything appears to be stored in ~/Library/Containers/com.apple.cloudphotosd/ and not in Application Support once you open the new Photos app and allow it to migrate things.

If you're in that situation (open the Photos app once and letting it set up a library and choosing which Photos to store locally), you should be able to confirm that nothing in iLifeAssetManagement is needed anymore and safely delete it after making a backup offline just in case we don't know all the tricks Apple has pulled.

Be sure to open iPhoto one last time and see if you need any of the photos there. If not, consider deleting that app and then converting all the iPhoto albums into Photos app albums and deleting the iPhoto Libraries once you have also an archival copy of these Libraries for "just in case" needs stored offline.