Thanks for your answer @Nash
Yes, most of our devices are Apple. I was a bit suspicious that the traffic I saw was downloaded, not uploaded. The iCloud traffic is marked as iTunes traffic and that was a bit unexpected for me, and moreover most traffic coming from Apple is marked as iTunes traffic. For anyone interested in this topic, I want to point out that apps downloaded from AppStore, documents, pictures, messages from desktop stored in iCloud will be marked as iTunes traffic. Thank you for your indication, it helped me understand how some settings/accounts work on Apple devices.
Most probably I will create some rules to shape this traffic and optimise the Wireless overall.
Thank you!