Unfortunately you don't fully understand how avast works.
The avastSvc.exe is the main avast service, it controls the various avast shields and crucially the localhost proxy for the web shield. All http traffic is routed through the web shield proxy, so it isn't actually avastSvc.exe that is initiating this bandwidth.
You should notice that your firefox bandwidth is considerably reduced also, you, via firefox want to browse the internet and that http traffic is routed through the proxy and comes back in through the proxy (so the content can be scanned) and then to your browser cache so it can be displayed in your browser.