Large file size of Storyline project file

I've got a Storyline project file which contains 8 slides. Most slides are a few shapes and some simple interactions/quizzes.

One slide has some imported jpeg graphics. When I look in the media library they total up to around 1MB.

The published zip file is 1.7MB, the exported folders are 6MB. Yet the Storyline project is 65MB! Where does that come from? Has anyone else experienced such large file sizes for projects with so little inside? Is there a way to tell where its coming from?