"System" Space usage increases everyday?

Before answering your question I would like to ask were are your other 36gb of hard drive storage if that is the 128gb storage option ? It is showing you only have a 86 gb drive space , Macs haven't had that low storage space for many years .This situation is many times due to temp folders writing excessive copies to the drive . You could go through your system and clear out the temps and cache files that are not needed from programs you no longer use or have . My wife's MacBook Pro had the same problem of "other" files taking up substantial amount of space . It was do to a program that wrote constantly temp files that were not needed or deleted even after the program was uninstalled . Have you backed up everything ? If so maybe a clean install of the os may be an option for you . It would freshen the system , rid of any clutter built up in it , and you could partition and install your windows os with out much complications .

If you are on High Sierra, see About Time Machine local snapshots - Apple Support .