Good detective work. You picked up a client that was using your account for massive spam forwarding so don't be surprised if some email recipient's ISPs have your IP address blocked.
The two things other than hijacking that really run up data usable are cloud apps that automatically back up everything to the cloud. Be wary of cloud backup particularly if business or creative work is uploaded. Terms of Service gives them a license for use of your photos.Facebook and Google both license your photos through stock image companies. They have the hi res versions so those become income streams for them and not you.
Another killer of bandwidth is Windows 10 and the later versions of Mac OS that generate constant uploading of all your activity on the computer. There are about 70 apps running in background communicating with MS about every search, keystroke and web page. MS and Apple got jealous of FB and Google who both compile massive amounts of data about you, your activities, friends etc and sell it or give it to the government. That is their main income stream. Now Microsoft has gotten into the action by monetizing your entire life. Using any social media or modern operating system other than Linux means you have zero privacy and that information is shared with all who buy it or its given to governments. Every email, social media post or IM goes into your record and AI applications read and analyze the content. Even your photos are analyzed to identify anyone in the photos. All this takes lots of data transmission in the background. Some countries are fighting back and blocking these privacy violations. I know here in Russia it is illegal to gather and store personal information about citizens by foreign entities. They have been battling FBI in court for some time with threats or banning FB over gross privacy violations so FB has tried workarounds to gather that info but not directly. As Snowden says, if you want privacy disconnect from the web entirely, otherwise every bit of data is vacuumed up and every phone call is captured by the NSA, for the entire world's data networks. You are the most spied on and monitored people in history. That law that permitted total surveillance was just renewed by Congress without debate yesterday.