They don't need to. They can infer based on the size and timing of the transfers. Nothing for a while then suddenly a huge download? Probably a picture. If the sizes are unique enough they might even figure out which.
But how do they know what you're looking at and how are they using this information to do anything useful? I get you can probably differentiate between text and video, but I'm having trouble understanding exactly what information they would get from your bandwidth consumption.
If there's an image of size X and they observe a download of X bytes, they can infer you downloaded and possibly looked at that image. The file size alone might be unique enough to allow that.
What they do with this information is anyone's guess. Just viewing something could put you into some kind of government watchlist. They could use parallel construction against you.
https://en.wikipedia.org/wiki/Traffic_analysis
To stop this, we'd have to saturate the link 100% of the time even when no useful communications are taking place.