Why Google Play services upload nonstop 1MB/s everytime the device is connected to wi-fi? Device: Nexus 5, Android Version 6.0.1 

Every time the phone is connected to my home wi-fi after a couple of minutes the phone uses my whole bandwidth to upload something to varying Google (1e100.net) Servers. I checked the Network Monitoring app and it tell me that the bandwidth is used by Google Play Services. 

The Uploading has been going on for three days nonstop and the only way to stop it temporarily is to disable and re enable the wi-fi connection.

What is causing this, and how do I stop it?
I think you are infected  with a malware that spy on your phone and upload to a comand and control server almost everything from your phone like: pictures, sms, whatsapp conversatin, facebook converstion, viber conversation etc, and to not affect your mobile data plan it starts the comunication with the server whenever you are connected to a wifi network...
Im not a pro on android but I've done it before and I already know what happens behind, how it works and how it affects your phone.
Do a backup but dont backup apps and after it try a factory reset.
Install the list of apps u had from Play store again 
(Its better to backup only pictures/videos and contacts and nothing more)
I hope it helps u.