We at support find that in case of "unusually large" files it's typical that drawings (on layouts) account to up to 80-90% of the file size. If the "Store drawing in the project file" option is checked in Drawing's settings, then all the individual lines and fills that make up the drawing is stored in the file, which can be big, depending on what you actually have in those drawings. Keep this checkbox off, if you don't mind waiting a few seconds while the drawings rebuild themselves when opening the layout. That may reduce the file size significantly.
Also, delete unnecessary "working" sections and elevations if you don't need anymore. That also boosts files size.
Thirdly, watch out for "dense" DWG content imported in your projects. Any 2D data (thousands of individual lines) can make your file big.
Fourthly, if there are embedded objects in the file, watch out for their size and remove any unnecessary objects.
Gergely Kmethy
VP, Customer Success, Graphisoft