6 gb file? That’s insane. I’m sure you must have gotten a warning? This happens if you use the lasso to pick up parts and stamp them again NUMEROUS times in THE SAME drawing. If you keep doing it, Animation Paper will be very slow when picking up and stamping (only then) and it will take up a lot of memory (and file size) as you experienced. Again, only if you keep doing the same thing beyond this warning.
This is partly by design – but something we need to address and optimise.
To reset your bad drawing (or drawings), you now need to trace (re-draw) them onto a new blank frame. Don’t use the lasso to copy. It will just copy the problem to the new frame.