In my case I have found that:
When backing up WhatsApp (on iOS - iPhone) chats to iCloud, it copies the backup into every machine that uses iCloud. The backup is placed into a hidden folder, on my machine (OS X 10.11.4) it is in the
folder, where UUUUUUU is my username and NNNNNNNNN is my phone number.
Unfortunately the backup file for the media is in one huge archive (mine is > 560MB) so every time WhatsApp backs up, the WHOLE file is re-downloaded, from what I can see. This is what nsurlsessiond is doing.
If you run the brctl log -w command mentioned above, you will see something along these lines:
received a push for container 57T923XXXX~net~whatsapp~WhatsApp
[note] 2.335 [2016-05-04 12:44:54.114] cloudkit.operation.callback sync-down.container-metadata BRCContainerMetadataSyncDownOperation.m:229
fetched 0 containers metadata from the cloud
[note] 2.375 [2016-05-04 12:44:54.154] sqlite.serverTruth zone.server BRCServerZone.m:771
received 25 edited items from the cloud for 57T923XXXX.net.whatsapp.WhatsApp
[note] 2.427 [2016-05-04 12:44:54.207] bird.scheduler.Apply Changes accountsession BRCAccountSession.m:1473
we will now download new documents automatically because the account contains less than 5 GB
[note] 2.536 [2016-05-04 12:44:54.316] sqlite.clientTruth fs.downloader BRCFSDownloader.m:1469
downloading 11 documents in 57T923XXXX.net.whatsapp.WhatsApp
Deleting the WhatsApp 'documents' (i.e. backups) via the iCloud storage management console in iOS or Mac OS X deletes the files from the Mobile Documents folder and thereafter there is no more activity from nsurlsessiond.
Hope this helps.