I also encountered the same issue. Using SpectraLayers for audio separation in ARA mode significantly increases the size of project and backup files. The test project only has one audio track and the corresponding SpectraLayers ARA, without any VSTi or VST effects.
I am using Cubase 13 Pro and SpectraLayers 12 Pro.
The result is determined by your Cubase project settings.
For example, if your source audio is 44.1 kHz/16 bit and your Cubase project setting is 192 kHz / 32bit, then the resulting audio files will be approximately 9 times the size of the original (192/44.1) x (32/16).
Try to Remove Extension from all processed files. This is what I do in Windows, and it works.
Or make the extension permanent. That should also do the trick, but I have never tried it being so used to using the Remove Extension option.
Best,
Magnus