Surprise, surprise: According to a 2016 article, "on average, almost half of the data required to view a web page is consumed by advertising," https://www.nextpit.com/ads-consume-half-of-your-mobile-data. The study is terrible; the result, believable. In fact, it must be much worse by now.
@zillion yes, Terrible study. I can likely make a more professional one in a couple hours. Conclusions are credible, but just because it fuels my confirmation bias.