I realize the drawing may be complex with hundreds of layers, 3rd party entities, lots of xrefs, etc., but ---- try this for starters
Make a copy of the file and work on the copy.
Open it up, delete all of the layouts, erase the viewport from the new layout it will create when you delete the last real one, then detach all of the xrefs, and erase everything from model space, then finally, purge it until you can't purge any more, then save it.
Now, what is the resulting file size?
I had a drawing the other day and after doing all of this, the file size was still 3MB. Obviously a lot of non-graphical bloat in the DWG file.
In my case, I opened it in Civil 3D (which was the origin) and Exported to ACAD, and the resulting export was 50k, like a blank drawing should be.
You may be able to open the file in BricsCAD or DraftSight and exporting out also