Hello,
since our Web Shield and Mail Shield process scans your http and mail traffic, we route your mail and web traffic through it. This means that this traffic shows at least once (communication between Avast and the web/mail server) and possibly twice (communication between browser/mail client and Avast) in Activity Monitor. The traffic attributed to Avast is most likely simply the browser and mail traffic.
While Avast communicates with our back end servers, the traffic should be nowhere close to the numbers you have mentioned.
Kind regards,
Ondrej Kolacek