Space of MacOS Mojave system

This is a question many of us have been pondering for some time without any clear answers but here are some possibilities:

1. The storage report is based on Spotlight's index and it isn't uncommon for the index to become munged. Rebuilding the Spotlight index might change things. How to rebuild the Spotlight index on your Mac - Apple Support
2. TimeMachine takes snapshots when the TimeMachine drive isn't available and stores them on the internal drive. These are System Files. Snapshot is supposed to delete snapshots that are more than one day old and unless you've installed new software or updated software a snapshot should be pretty small. The snapshots should be deleted once TimeMachine has performed its backup.
3. You may have cache files that need to be trimmed or deleted. This should be performed at regular times by the OS but sometimes this doesn't occur. Restarting in Safe Mode will do this for you. Use safe mode to isolate issues with your Mac - Apple Support

This is only a partial answer. Poking around on my work desktop I did the above and reduced my System from about 90GB to under 70GB a few weeks ago. It remains that size now. But it still shouldn't be this big...I think. Maybe others on the ASC site have more to add.