I should note that upon further inspection I found that it doesn't appear to be as bad as I thought. The largest chunk of this 26.52 GB archive is taken up by the following.
Youtube - 9.45 GB
Mail - 4.72 GB
Google Photos - 10.2 GB
Drive - 3.79 GB
This is all stuff which I've stored there myself over the course of time. There is of course a whole bunch of meta-data, but that hardly takes up any space in the archive compared to the files.
So after subtracting the above files from the total archive, that means that Google has collected
-1.64 GB of data on me. That's right, that's a minus-sign. Go figure!