Without seeing the file, it’s almost impossible to tell, but judging from the screen grabs, I’m seeing a lot of very dense isocurves; indicating a lot of complex surfaces resulting in a lot of complex chamfers. If the render mesh is set really fine, it can get quite large. Try and SaveSmall to see the file size for the NURBS geo only - although that will require a new render mesh to be generated when you shade the viewport, once again increasing the file size.
if you still not find why the file is so big:
copy half of the geometry to a new file - check file size
do it a again visually half of the geometry to a new file → size ?
always repeat with the bigger file… this will somehow isolate some to dense geometry.