I had similar problems - for example when creating a template file. Looks like Archicad keeps some data even after all model elements were deleted.
It can be drawings, placed into layouts (when everything is deleted from model space, auto-generated drawings placed into layout can still take space until this drawing was updated or deleted). Probably it can be calculation data - schedules, that already been proceeded once.
Hiya - my first instinct was also the Embedded Library but as noted above it is empty.
However I see there are lots of stories in the project - while empty this creates a large "space" for Archicad to think about. I tried deleting all of the sections and elevations (again they are empty but they have to "show" the space) and the file size dropped to 11MB. In our experience live sections and elevations are notorious for increasing file size (especially Teamwork projects when there are multiple people creating these viewpoints)