Some interesting observations there.
I created the CAD model drawn in inches so I'm not sure why it had to be resized.
Could it be when you import the file to your slicing software that it's default import settings are metric & would need to be changed to inch units?
I can't explain the file size difference, when I created the STL file I just used the default settings. The model did appear "coarse" when I import it back in. I can tweak those settings if need be.
If you would like, I can send you a STP file to import into your CAD program so you can play with the export settings & see what works for you.
John