nsurlsessiond downloading 100s GB in a month
Whatsapp was causing it. Whatsapp's internal backup system to iCloud Drive. If you have have your Mac setup to sync with iCloud, it will prefetch all iCloud drive content to your Mac as cache, which includes your Whatsapp Backup. Since your whatsapp backup are constantly renewed, the Mac will constantly have to fetch the newer version. 

For me the Whatsapp backup was 5GB, which is why it easily consume all your bandwith. This problem could also be the same for any other Apps that backup to iCloud. Disable the Whatsapp Internal iCloud Drive backup was the solution, as whatsapp itself is backed up when you do full iPhone Backup to iCloud.

Problem has been reported to both Apple and Whatsapp few weeks after this post, in 2016 i tested it and the problem was still there. Not sure about it now.