Ever 3rd (ca) thing I hear on Spotify is an ad for Spotify Premium
Ever 5-6 minutes on DI, I get an ad
On Sony's music-video service I got 2 ads per song (Often the same twice)
On YouTube I get a video-ad per video, easily as long as the video I'm watching in case of short "funny" vids (I've clocked youtube video-ads up to 3 minutes long)
On Comedy Central, watching Daily Show and Colbert Report I get (got?) 1-3 video-ads per ca 6-minute segment
The notion that stripping this could result in 25% reduction seems... high, but plausible, especially if the study went for ad-heavy traffic (it was funded by AdBlock Plus)
Then on text/image sites, you get large anim-GIFs, multiple sizable javascripts, and flash elements, that together can take up more than half the data transferred... so averaging 25% becomes likely I think.