That's by design to keep connections with other peers.
Some kind of throttle would be a good idea, in that case. A gigabyte a day is unacceptable for some of us.
That is a fundamentally broken design. There is literally no excuse for a simple messenger app to suck up a gigabyte daily. How do you expect people to adopt this when they have broadband and wireless plans with data caps?
I know nothing about TOX design, but it makes (to me, at least) some privacy sense to saturate the network with noise that is in message length, their interarrival time and recipient characteristics similar to the actual communication. No idea if TOX does that.
Aside from the data usage - that will suck the battery life from your devices as well
To get back on this, this might be only the Windows client, just had latest qTox idling on Linux overnight and network data usage is 1.3MB.