turn off (or "down") autosave!! and delete the files -- or get an external drive.
At least check your autosave settings.
1) You can find out where autosave is saving backups in Preferences > Cache > Autosave
reset the Keep at Most: to about 3 - 5 (if Motion crashes, you usually only need one!)
2) You can go to the path listed in Autosave and dump a lot of the older files
PS - you can also delete the Retiming cache...
I've done that. The auto-save folder equaled about 360mb. This hard drive hogging seems to only occur as it is exporting a file. But I am not sure why the program requires almost 64 gigs of hard drive space to export a file! I did a similar action using FCPX and never ran into a single problem (and that was an 8 minute video with tons of transitions and filters).