Incomplete TimeMachine backups are taking up disk space on my Mac.

My MacBook (macOS Mojave, happened on other OS too) keeps shrinking on space and even after a restart I can't seem to reclaim most of it. Of the 256 GB HDD, ~173 GB is taken up by the "system" with only <7 GB free. I also have a time capsule that wirelessly backs up my MBPr.

Recently, I restored my mac from a backup and reclaimed over 50 GB free space.

It all worked fine till I had to leave my mac away from me for a few days. When I returned, I realized that the backups didn't happen and the free space is now down to 7 GB. Even restarting didn't help.

Hypothesis: I suspect that the Time Machine backups that don't complete successfully are still stuck in the system and need to be flushed out.

Question: How do I do this?