Please explain why the app needs so much bandwidth with zero file transfers

A decentralised network is one made up soley of its users, there is no web/central server to manage or organise the users so this has to be undertaken by each client itself, this then is called the "network overhead", various network have a different value for this depending on such things as the protocol in usage the quantity of fellow users and how the client itself is designed to facillitate interaction following any hard-coded internal limits.

All decentralised networks will have this overhead and some secure networks will even add fake traffic (spoof traffic) to make it harder for anyone trying to break into the network by doing traditional message packet traffic analysis.

Theres a lot more going on than just file transfers.